[DATAFU-182] feat(spark): add Spark 3.5 support

[InbalHoni]

Summary

Adds support for Spark 3.5.x by extending the Scala 2.12 build and test matrix
through Spark 3.5.9, updating the compatibility documentation, and moving CI
to Python 3.8 as required by Spark 3.5.

Validation

Before opening this PR, I manually ran the tests workflow from feature/ spark-3.5 on my origin fork. The workflow completed successfully .

[eyala]

It looks good! One question - if you didn't change any of the actual source files, only changed the Python version used in testing, maybe that means that version 2.1.0 does support Spark 3.5.x, and we can update that in the README instead of 2.2.0

[InbalHoni]

It looks good! One question - if you didn't change any of the actual source files, only changed the Python version used in testing, maybe that means that version 2.1.0 does support Spark 3.5.x, and we can update that in the README instead of 2.2.0

Right, the source files are effectively unchanged. 2.1.0's declared range was 3.0.x–3.4.x, so the question is whether we want to retroactively extend that declared range to include 3.5.x, or bump to 2.2.0 and give that the wider range.