[21230] Migrate Discovery Server CI to Github (backport #90)

[mergify]

Description

This PR improves the GitHub CI of this repo, to make it as similar as the one in Fast DDS

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

• Remove nightly jobs
• Update ubuntu-ci.yml file with the proper fallback tag
• Change fastdds.repos to fastrtps.repos
• Fast DDS 2.6.x requires building on ubuntu-20.04

@Mergifyio backport 1.2.x

Contributor Checklist

• Commit messages follow the project guidelines.
• N/A The added tests pass locally.
• N/A Changes are backwards compatible.
• 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.

---

This is an automatic backport of pull request #90 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of c323886 has failed:

On branch mergify/bp/1.2.x/pr-90
Your branch is up to date with 'origin/1.2.x'.

You are currently cherry-picking commit c323886.
  (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/build.meta
	new file:   .github/workflows/config/test.meta
	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/pull_request_template.md

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