PYTHON-5522: Support std lib zstandard in 3.14

[Rogdham]

Issue Key

Summary

Python 3.14 introduces support for Zstandard in the standard library via the compression.zstd module (PEP-784).

This PR removes the zstdandard dependency in favor of the native support in Python 3.14+. For older Python versions, the backport is used.

Full disclosure: I'm the author and maintainer of backports.zstd, and the maintainer of pyzstd (which code was used as a base for the integration into Python). I also helped with PEP-784 and its integration into CPython.

Changes in this PR

• Remove zstandard dependency (in the extra group zstd)
• Add backports.zstd dependency for Python before 3.14 (in the extra group zstd)
• Adapt code accordingly

Testing Plan

I tried to run the tests but fail to launch just setup-tests, so all tests were not run.

I let reviewers perform tests on both Python versions before 3.14 and 3.14.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created?

Checklist for Reviewer {@primary_reviewer}

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?

Focus Areas for Reviewer (optional)