[19696][20409] Fix data race on PDP (backport #4220)

[mergify]

This is an automatic backport of pull request #4220 done by Mergify.
Cherry-pick of 3053832 has failed:

On branch mergify/bp/2.6.x/pr-4220
Your branch is up to date with 'origin/2.6.x'.

You are currently cherry-picking commit 305383295.
  (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:   include/fastdds/rtps/attributes/ServerAttributes.h
	modified:   src/cpp/rtps/builtin/discovery/participant/PDPClient.cpp
	modified:   src/cpp/rtps/builtin/discovery/participant/PDPListener.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   test/blackbox/common/DDSBlackboxTestsDiscovery.cpp

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

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[Mario-DL]

@juanlofer-eprosima @Tempate would you mind taking a look at the conflicts ?
Also, take advantage to rebase, so that we have both windows & mac on github working for this branch

[Mario-DL]

@Mergifyio rebase 2.6.x

[mergify]

rebase 2.6.x

✅ Branch has been successfully rebased

[Mario-DL]

@richiprosima please test this

[Mario-DL]

@richiprosima please test windows

[Mario-DL]

The scarce failed tests are unrelated, ready to merge !