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

[mergify[bot]]

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

• [x] 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.

• [x] Applicable backports have been included in the description.

Reviewer Checklist

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

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