[21226] Improve CI workflows in 2.1.x, and branching model refactor (backport #365)

[mergify]

Description

This PR improves the CI workflows, making them as similar as possible to the Fast DDS workflows.
It also introduces a braching model refactor, similiar as the performed in Fast DDS repository.

Note: While backporting this PR, please ensure the following changes:

• Remove nightly jobs
• Update ubuntu-ci.yml file with the proper fallback branches
• Change fastdds.repos to fastrtps.repos
• 3.3.x and 3.2.x requires both 1.1.x and 2.x Fast CDR branches to be tested
• 2.5.x requires only jdk11
• 2.1.x requires building on ubuntu-20.04, and only jdk11

@Mergifyio backport 3.3.x 3.2.x 2.5.x 2.1.x

Contributor Checklist

• Commit messages follow the project guidelines.

• N/A Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally

• N/A New feature has been documented/Current behavior is correctly described in the documentation.

• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.

---

This is an automatic backport of pull request #365 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of d912d11 has failed:

On branch mergify/bp/2.1.x/pr-365
Your branch is up to date with 'origin/2.1.x'.

You are currently cherry-picking commit d912d11.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   .github/workflows/config/test.repos
	new file:   .github/workflows/nightly-ubuntu-ci.yml
	new file:   .github/workflows/reusable-ubuntu-ci.yml
	new file:   .github/workflows/ubuntu-ci.yml
	new file:   RELEASE_SUPPORT.md

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   .github/workflows/config/build.meta

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[JesusPoderoso]

LGTM with green CI

[JesusPoderoso]

LGTM