Fix OSS builds (removes 3.12mp build)

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I think it's reasonable to not have an OSS build of 3.12 - in practice nobody is ever going to use this oddity as it requires Meta Python. CinderX with stock CPython is what we expect going forwards.

I'm trying to reconcile a few constraints here without making the code too complicated:
* For PGO with LLVM we need to have llvm-ar installed, but this isn't available on Sandcastle or Ubuntu Latest (on GitHub).
* It's easyish to install GCC on Sandcastle.
* GCC-13 is available on Ubuntu Latest in GitHub but installing another version is annoying because our GitHub workflow for getdeps is autogenerated.
* GCC-13 is sufficient for CinderX + 3.14 but GCC-14 is needed for CinderX + 3.12 (something seems to have been fixed around headers for atomics when mixing C + C++).

So, making GCC-13 a minimum requirement for OSS builds is good enough to get PGO builds working on Sandcastle + GitHub provided we only target 3.14+.

Reviewed By: itamaro

Differential Revision: D88570106

fbshipit-source-id: 7151be4bda9f754d296bf56f20fba90615918a54

Author: jbower-fb