Bump FreeBSD platform requirements to v14+

[rami3l]

Mirrors rust-lang/rust#156607.

Anything regarding FreeBSD v13 or lower is now unsupported on CI. However as explained in rust-lang/rust#156607 (comment), CI builds targeting v14 should continue to work on v12 and v13 for now.

Tasks

• ci(docker/freebsd): bump clang version to freebsd14 #4896
• ci(freebsd): use FreeBSD 14.0 for full CI #4750 (to be repurposed to use v14 instead of v15)