Bump 510 workflows to the 5.1.0 release

[jmid]

As the title says, this PR bumps the 510 workflows to the 5.1.0 release.

I also spotted that the 5.1.0 opam install workflow was using 5.1.0+trunk - which is marked as not available with with 5.1.0 release causing it to fail. The PR therefore switches to 5.1.0 there too and adds an entry for 5.2.0+trunk.

Finally, I snuck in an unrelated update to mark the framepointer Effect crash issue as fixed.

[jmid]

CI summary:

• Cygwin 5.1 part1 timeout during Lin DSL In_channel test with Domain with multiple +2000s tests [ocaml5-issue] General Cygwin slowness #384
• MingW bytecode trunk crash in threadomain [ocaml5-issue] Windows failures on threadomain #203
• linux-arm64-5.1 failed to install 5.1.0+trunk
• linux-s390x-5.1 failed to install 5.1.0+trunk
• linux-ppc64le-5.2 triggered sequential STM Bytes and Float.Array crash/abort [ocaml5-issue] Crashes and hangs on ppc64 trunk/5.2 #380

Of the 60 completed runs 5 failed: 2 failures were genuine, 1 was a false alarm, and 2 due to a CI-setup issue.
I'll merge.

[jmid]

CI summary for merge to main:

• MingW trunk timeout during threadomain [ocaml5-issue] Windows failures on threadomain #203
• macOS trunk failed to trigger STM int ref test parallel asymmetric in 5000 iterations 'STM _ ref test parallel asymmetric' failure to trigger #364
• linux-ppc64le-5.2 crashes/aborts during sequential STM Bytes, Array, and Float.Array tests [ocaml5-issue] Crashes and hangs on ppc64 trunk/5.2 #380

Out of the 39 completed runs 3 failed: 2 genuine, 1 gray zone