Allow backported bugfixes to picked up by CI/CD

[CodeGat]

In this PR:

• Added pull_request trigger for backport/*.* branches in all workflows. See the linked issue for an explanation of the new branching scheme.
• Removed the restriction of pre-VERSION branch naming. We now get VERSION from the spack.yamls access-om2 spec.
• ci.yml:
  • Restructured job dependencies
  • prerelease-deploy-version job now yqs spack.yaml to get the versions
• cd.yml:
  • Simplified getting the tag name by just using the spack.yamls access-om2 spec version.
  • Removed the Prerelease cleanup, now in a new closed-pr.yml.
• Added a closed-pr.yml workflow for Prerelease cleanup after PR.
• Added a CONTRIBUTING.md and referenced it in the README.md.

Closes #50