Switch GA workflows to test 5.1.1~rc1

[jmid]

Following the release of 5.1.1~rc1 this PR updates our 5.1.x workflows to use it.

The PR also renames the workflow files from using the substring 510 to 51x for consistency and to avoid having to rename later (I don't expect there to be a 5.1.2 though).

[jmid]

CI summary:

• Linux 5.1 debug aborted with an assertion failure in threadomain [ocaml5-issue] Assertion failure on threadomain under debug runtime #404
• linux-s390x-5.2 timedout after multiple long runs (1635.8s Domain.spawn/join - atomic, 3081.5s Lin Out_channel test with Domain, 1149.1s Lin Atomic test with Domain, 3772.9s STM Weak test parallel)

Out of 63 workflows 2 failed with one genuine and one borderline reliability/OCaml5 issue

[jmid]

CI summary for merge to main:

• linux-s390x-5.2 timed out and was cancelled after several +1000s test runs (1311.4s Domain.spawn/join - atomic 1191.7s Lin Out_channel test with Domain 1299.1s Lin Bigarray.Array1 (of ints) test with Domain 2186.5s Lin Atomic test with Domain 2017.8s Lin Hashtbl test with Domain)

Out of 38 workflows, 1 failed - a borderline reliability/OCaml5/CI issue