[CI] Add tests for LibreSSL 2.7

[straight-shoota]

This is the oldest version of LibreSSL I have been able to dig up. It works in a modern Alpine image, but there's a conflict which leads apk add to exit with a failure code. It's still fine to use though. But this means we need to ignore the status code.
To validate we're still using the intended package version, I added a simple check for the version constants in Crystal. So this is even more robust than before.

[ysbaddaden]

LibreSSL 2.7 wasn't updated since December 2018.

Is there really any worth into adding to CI in 2024?

[ysbaddaden]

Reading https://www.libressl.org/releases.html I understand that only 3.9 (released with OpenBSD 7.5) and 4.0 (released with OpenBSD 7.6) are currently supported, while 3.8 is EOL since it was released with OpenBSD 7.4 that itself reached EOL a month ago.

[straight-shoota]

We have conditionals in the bindings to deal with LibreSSL < 3.0. #15177 is adding even more.
So there's some merit in having this code actually tested. And it's not too much extra effort to do so.

The test for LibreSSL is just another matrix entry and ignoring errors of apk add. So it's not much.
The Check LibSSL version step is useful anyway.

Of course it's unclear how long we this will work that we can install libressl 2.7 from Alpine 3.9 in the latest Alpine image. We can drop it as soon as it breaks. But while it works, it adds a bit of extra comfort that the code that we have is working.