[20553] Set 2.12.x as EOL

[JesusPoderoso]

Description

This PR marks 2.12.x as an end-of-life version, by:

• removing it from backports list in the pull request template
• removing it from the nightly CI
• removing the bold mark in the supported versions MD

Contributor Checklist

• Commit messages follow the project guidelines.
• N/A The code follows the style guidelines of this project.
• 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 Any new/modified methods have been properly documented using Doxygen.
• N/A Changes are ABI compatible.
• N/A Changes are API compatible.
• N/A New feature has been added to the versions.md file (if applicable).
• New feature has been documented/Current behavior is correctly described in the documentation.
  Related documentation PR: [20553] Set 2.12.x as EOL Fast-DDS-docs#733
• N/A 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.

[JesusPoderoso]

@richiprosima please test this

[EduPonz]

I think this PR should also remove 2.12.x from:

• Nightly CI
• RELEASE_SUPPORT.md (should not be bolded anymore)