[17976] Correctly assign multicast port to multicast initial peers (backport #3425)

[mergify]

This is an automatic backport of pull request #3425 done by Mergify.
Cherry-pick of 49baf51 has failed:

On branch mergify/bp/2.1.x/pr-3425
Your branch is up to date with 'origin/2.1.x'.

You are currently cherry-picking commit 49baf5154.
  (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:
	modified:   test/blackbox/common/BlackboxTestsDiscovery.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/cpp/rtps/transport/UDPTransportInterface.cpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[jsan-rt]

@richiprosima please test windows test mac

[jsan-rt]

Windows test failure is unrelated.

[jsan-rt]

Linux was tested locally with no related test failures.