Use backports.zstd for zstandard compression

Python 3.14 introduced [native zstandard](https://docs.python.org/3/library/compression.zstd.html) support, and there is now a [backports.zstd](https://pypi.org/project/backports.zstd/) package that we can use for older Python versions. 

Change zstd compression support in flow.record to use `backports.zstd` instead of the `zstandard` library that we use now.