Separate workflows for the 5.2 branch

[jmid]

This PR

• adds separate 5.3+trunk workflows to follow trunk
• updates the 5.2+trunk workflows to follow the 5.2 branch on ocaml/ocaml

Finally, to reduce CI CPU cycles, it adjusts the yaml-workflows so that pushes from me do not trigger double runs for both the push and for the PR.

[jmid]

To fix the MinGW and Cygwin failures on 5.2 and 5.3 I've filed https://github.com/shym/custom-opam-repository/pull/8

[jmid]

CI summary for 4c437bc

• All 6 Cygwin+MinGW 5.2 and trunk workflows failed because of version mismatch after the 5.2 branch and the trunk bump to 5.3

Out of 44 workflows 6 failed due to CI errors

[jmid]

I just force-pushed an update to trigger a clean rerun after shym/custom-opam-repository#8 has now been merged

[jmid]

CI summary:

• 32bit 5.2 / build / test (pull_request) Failing after 46m unexpectedly found a counterexample in Lin.Internal CList int test with Thread #358
• Cygwin 5.1 timed out during negative Lin Dynlink test #307
• 6 workflows aborted domain_spawntree with Fatal error: Failed to create domain #428
  • Cygwin 5.2
  • Cygwin trunk
  • MinGW 5.2
  • MinGW bytecode 5.2
  • MinGW bytecode trunk
  • linux-arm64-5.2

Out of 44 workflows 8 failed with 7 genuine failures and 1 false alarm.

[jmid]

CI summary for merge to main a571972:

• 5 workflows aborted domain_spawntree with Fatal error: Failed to create domain #428
  • Cygwin 5.2
  • Cygwin trunk
  • MinGW 5.2
  • MinGW bytecode 5.2
  • linux-s390x-5.2 (failed: Failed: Build failed)
• MinGW trunk failed without any error log available!

Out of 45 workflows 6 failed with 5 genuine failures and 1 ci system defect.