[22841] Improve max_allocations calculation on SHM transport (backport #5659)

[mergify]

Description

This PR improves SHM transport throughput by changing the calculation of the maximum number of allocated buffers.
It will now be the maximum between segment size / mean_message_size and the configured port_queue_capacity.

@Mergifyio backport 3.1.x 2.14.x 2.10.x

Contributor Checklist

• Commit messages follow the project guidelines.

• The code follows the style guidelines of this project.

• NO: 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

  • I couldn't find a way to make a regression test, other than running a complex scenario and manually inspecting the results.

• 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)

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

• 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.
• If this is a critical bug fix, backports to the critical-only supported branches have been requested.
• 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 #5659 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of b54cb8e has failed:

On branch mergify/bp/2.10.x/pr-5659
Your branch is up to date with 'origin/2.10.x'.

You are currently cherry-picking commit b54cb8ef.
  (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:   include/fastdds/rtps/transport/shared_mem/SharedMemTransportDescriptor.h
	both modified:   src/cpp/rtps/transport/shared_mem/SharedMemTransport.cpp
	both modified:   src/cpp/rtps/transport/shared_mem/SharedMemTransportDescriptor.cpp
	both modified:   test/mock/rtps/SharedMemTransportDescriptor/fastdds/rtps/transport/shared_mem/SharedMemTransportDescriptor.h

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

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