CI updates

[jmid]

This PR lumps 3 CI updates into one:

• The first 2 commits removes the 5.0 workflows (and badges). After the merge of the Gc tests Add gc tests #469, 9/9 of them failed the scheduled run, creating lots of notification email noise. Rather than spending time adjusting them to accept misbehavior on 5.0, now seems a natural point to stop running them and save some CPU cycles in the process.
• The next 2 commits removes the 5.1 musl workflow (and badge) which is failing compilation with zstd.h not found. This is a known bug and the opam 5.1.1 opam package was explicitly back-patched in Backport ocaml/ocaml#13100 to 5.1.x ocaml-variants ocaml/opam-repository#25828 to handle this error. In choosing between continuing to fail, spending time on patching in our opam-free setup, and removing the musl-workflow from 5.1, the latter seems most sane.
• The last commit updates the gh-pages workflow to also use setup-ocaml@v3 - an oversight from Use setup-ocaml 3 for opam workflow #512

[jmid]

CI summary for 91072aa

• Cygwin trunk timed out after 7529.2s shrinking in Lin Dynarray test with Domain Timeout due to excessive shrinking in Lin Dynarray test with Domain #514

Out of 34 workflows, 1 failed with a false alarm.

[jmid]

CI summary for merge to main: all 35 workflows passed!