Prevent stdlib compat entries from breaking old Julia versions

[DilumAluthge]

In old Julia versions, Pkg.test sets the version numbers of all stdlibs to 0.0.0. So, if we don't allow 0.0.0, then Pkg.test will throw a resolver error.

Example of this breaking: JuliaRegistries/RegistryCI.jl#557 (e.g. this CI log).

See also:

1. https://github.com/JuliaBinaryWrappers/MozillaCACerts_jll.jl/blob/main/Project.toml
2. https://github.com/JuliaRegistries/General/blob/master/jll/M/MozillaCACerts_jll/Compat.toml
3. https://github.com/JuliaRegistries/General/blob/master/jll/M/MozillaCACerts_jll/Versions.toml

[giordano]

It just occurred to me that when this goes live in Yggdrasil, this will make all new builds of packages with the same X.Y.Z version number fail in the registry because the compat bound will have changed. I'm not particularly thrilled by having to manually fix all of them.