prepare-cpp_stl: downgrade to Boost 1.76 (from 1.85) on macOS

See the code comment.

For the record, this is what the compile errors in the CI job `macos
(cpp_stl, _98, clang14)` at
41ad89f
looked like (this would repeat 248 times, which matches the number of
`.cpp` files in
https://github.com/kaitai-io/kaitai_struct_tests/tree/390e0ee4bf817bfb86154a841213a45307a0fb1a/spec/cpp_stl_98):

```
[  1%] Building CXX object CMakeFiles/ks_tests.dir/test_bcd_user_type_le.cpp.o
/Applications/Xcode_14.2.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ \
    -DBOOST_UNIT_TEST_FRAMEWORK_DYN_LINK -DBOOST_UNIT_TEST_FRAMEWORK_NO_LIB \
    -DKS_STR_ENCODING_ICONV -DKS_ZLIB \
    -I/Users/runner/work/ci_targets/ci_targets/compiled/cpp_stl_98/bin \
    -I/Users/runner/work/ci_targets/ci_targets/tests/compiled/cpp_stl_98 \
    -I/Users/runner/work/ci_targets/ci_targets/tests/spec/cpp_stl_98/../../../runtime/cpp_stl \
    -I/Users/runner/work/ci_targets/ci_targets/tests/spec/cpp_stl_98/prereq \
    -isystem /usr/local/include -g -std=c++98 -isysroot \
    /Applications/Xcode_14.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.1.sdk \
    -mmacosx-version-min=12.7 -MD -MT CMakeFiles/ks_tests.dir/test_bcd_user_type_le.cpp.o \
    -MF CMakeFiles/ks_tests.dir/test_bcd_user_type_le.cpp.o.d \
    -o CMakeFiles/ks_tests.dir/test_bcd_user_type_le.cpp.o \
    -c /Users/runner/work/ci_targets/ci_targets/tests/spec/cpp_stl_98/test_bcd_user_type_le.cpp
In file included from /Users/runner/work/ci_targets/ci_targets/tests/spec/cpp_stl_98/test_bcd_user_type_le.cpp:3:
In file included from /usr/local/include/boost/test/unit_test.hpp:18:
In file included from /usr/local/include/boost/test/test_tools.hpp:54:
In file included from /usr/local/include/boost/test/tools/fpc_op.hpp:19:
In file included from /usr/local/include/boost/test/tools/fpc_tolerance.hpp:19:
In file included from /usr/local/include/boost/test/tree/decorator.hpp:22:
In file included from /usr/local/include/boost/test/tree/fixture.hpp:21:
In file included from /usr/local/include/boost/function/function0.hpp:10:
In file included from /usr/local/include/boost/function/function_template.hpp:16:
In file included from /usr/local/include/boost/mem_fn.hpp:22:
/usr/local/include/boost/bind/mem_fn.hpp:93:1: error: 'auto' not allowed in function return type
auto mem_fn( R (T::*pmf) (A...) ) -> _mfi::mf<decltype(pmf), R, T, A...>
^~~~
/usr/local/include/boost/bind/mem_fn.hpp:93:34: error: expected ';' at end of declaration
auto mem_fn( R (T::*pmf) (A...) ) -> _mfi::mf<decltype(pmf), R, T, A...>
                                 ^
/usr/local/include/boost/bind/mem_fn.hpp:93:35: error: cannot use arrow operator on a type
auto mem_fn( R (T::*pmf) (A...) ) -> _mfi::mf<decltype(pmf), R, T, A...>
                                  ^
/usr/local/include/boost/bind/mem_fn.hpp:99:1: error: 'auto' not allowed in function return type
auto mem_fn( R (T::*pmf) (A...) const ) -> _mfi::mf<decltype(pmf), R, T, A...>
^~~~
/usr/local/include/boost/bind/mem_fn.hpp:99:40: error: expected ';' at end of declaration
auto mem_fn( R (T::*pmf) (A...) const ) -> _mfi::mf<decltype(pmf), R, T, A...>
                                       ^
/usr/local/include/boost/bind/mem_fn.hpp:99:41: error: cannot use arrow operator on a type
auto mem_fn( R (T::*pmf) (A...) const ) -> _mfi::mf<decltype(pmf), R, T, A...>
                                        ^
/usr/local/include/boost/bind/mem_fn.hpp:196:5: error: 'auto' not allowed in function return type
    auto operator()( U&& u ) const -> decltype( std::forward<U>( u ).*pm_ )
    ^~~~
/usr/local/include/boost/bind/mem_fn.hpp:196:35: error: expected ';' at end of declaration list
    auto operator()( U&& u ) const -> decltype( std::forward<U>( u ).*pm_ )
                                  ^
8 errors generated.
gmake[2]: *** [CMakeFiles/ks_tests.dir/build.make:160: CMakeFiles/ks_tests.dir/test_bcd_user_type_le.cpp.o] Error 1
gmake[2]: Leaving directory '/Users/runner/work/ci_targets/ci_targets/compiled/cpp_stl_98/bin'
gmake[2]: Entering directory '/Users/runner/work/ci_targets/ci_targets/compiled/cpp_stl_98/bin'
```

prepare-cpp_stl

@@ -12,7 +12,20 @@ fi
 
 # On GitHub Actions for macOS, install prereqs from brew
 if [ -n "$GITHUB_ACTIONS" ] && [ "$RUNNER_OS" = macOS ]; then
+	# NB: since commit
+	# https://github.com/Homebrew/homebrew-core/commit/c67b591208748bb8d07e79806c4963bbe1a9c866 made
+	# on 2024-04-26, we can no longer use just `brew install boost`
+	# (https://formulae.brew.sh/formula/boost), because it got updated to Boost 1.85, which then
+	# broke our `cpp_stl_98/clang14-macos-x86_64` CI job. Apparently, Boost 1.85 apparently no
+	# longer builds with `-std=c++98`, since we were suddenly getting syntax errors in
+	# `/usr/local/include/boost/bind/mem_fn.hpp`. This matches the
+	# [changelog](https://www.boost.org/users/history/version_1_85_0.html):
+	# > Bind:
+	# > * C++03 is no longer supported; a C++11 compiler is required. (This includes GCC 4.7 or
+	# >   later, and MSVC 12.0 (VS 2013) or later.)
+	#
+	# Fortunately, https://formulae.brew.sh/formula/[email protected] is available, so we use that.
 	brew install \
-		boost \
+		boost@1.76 \
 		make
 fi