[15766] Update shared_mutex thirdparty to don't prioritize writers (backport #2976)

[mergify]

This is an automatic backport of pull request #2976 done by Mergify.
Cherry-pick of 47d85d3 has failed:

On branch mergify/bp/2.6.x/pr-2976
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Changes to be committed:
	modified:   cmake/common/gtest.cmake
	new file:   cmake/modules/check_shared_mutex_priority.cpp
	modified:   include/fastrtps/utils/ProxyPool.hpp
	modified:   src/cpp/CMakeLists.txt
	modified:   src/cpp/rtps/history/TopicPayloadPool.cpp
	modified:   src/cpp/rtps/participant/RTPSParticipantImpl.cpp
	modified:   src/cpp/rtps/participant/RTPSParticipantImpl.h
	modified:   test/blackbox/common/DDSBlackboxTestsBasic.cpp
	new file:   test/unittest/utils/shared_mutex_tests.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   include/fastrtps/utils/shared_mutex.hpp
	both modified:   test/unittest/utils/CMakeLists.txt

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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[MiguelBarro]

@richiprosima Please test this for me 🤯

[MiguelBarro]

@richiprosima Please test this for me 🤯

[MiguelBarro]

@richiprosima Please test this for me 🤯

[MiguelBarro]

@richiprosima Please test this for me 🤯

[MiguelCompany]

@richiprosima Please test this since there was another PR merged in the middle.