Use backports.zstd instead of zstandard

[cclauss]

Summary

• Use standard libary Zstandard for Python 3.14+ #3613 (comment)

Suggested using https://pypi.org/project/backports.zstd instead of https://pypi.org/project/zstandard to be more aligned with the zstandard capabilities being added to the Python 3.14 standard library.

• https://docs.python.org/3.14/whatsnew/3.14.html#whatsnew314-pep784

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.