[21158] Update README.md with GitHub actions Ubuntu CI nightly (backport #4979)

[mergify]

Description

This PR updates de Ubuntu CI nightly badge with the lately improved Github actions Ubuntu CI.

@Mergifyio backport 2.14.x 2.13.x 2.10.x 2.6.x

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 Any new configuration API has an equivalent XML API (with the corresponding XSD extension)

• N/A Changes are backport compatible: they do NOT break ABI nor change library core behavior.

• N/A Changes are API compatible.

• N/A New feature has been added to the versions.md file (if applicable).

• 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 #4979 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 1409dc2 has failed:

On branch mergify/bp/2.6.x/pr-4979
Your branch is up to date with 'origin/2.6.x'.

You are currently cherry-picking commit 1409dc245.
  (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)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   README.md

no changes added to commit (use "git add" and/or "git commit -a")

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

[EduPonz]

This does not look like the master one