[21790] Add references to new nightly jobs per supported branch in README (backport #5293)

Description

This PR adds references to new nightly jobs per supported branch in README

This PR is on top of (and should be merge after):

@Mergifyio backport 3.0.x 2.14.x 2.10.x 2.6.x

NOTE: references to master should be updated in the backports to the corresponding branch name

[x] Commit messages follow the project guidelines.
[x] 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.
[x] Applicable backports have been included in the description.

[x] The PR has a milestone assigned.
[x] The title and description correctly express the PR's purpose.
[x] Check contributor checklist is correct.
N/A If this is a critical bug fix, backports to the critical-only supported branches have been requested.
N/A Check CI results: changes do not issue any warning.
N/A Check CI results: failing tests are unrelated with the changes.
This is an automatic backport of pull request #5293 done by Mergify.