Change from `libc-test/semver/*.txt` to `cargo-semver-checks`

[tgross35]

These text files are often out of date and not always clear about how they get used. We should delete these all and just use cargo-semver-checks.

I believe release-plz runs this but only on Linux, we need to check all Tier 1 and (maybe?) Tier 2 platforms.

[tgross35]

Initial implementation at #4504, unfortunately it seems like we can only test native targets due to obi1kenobi/cargo-semver-checks#1068 / obi1kenobi/cargo-semver-checks-action#98.