Spark2 deprecation

[Zoltan Chovan <zc...@apache.org>]

Hi all,

I'd like to propose deprecating Spark 2 support in the Kudu Spark connector.

Apache Spark 2.x has reached end-of-life. The final 2.x release, Spark
2.4.8, was published in May 2021, and the Spark community no longer
provides bug fixes or security patches for the 2.x line. Spark 3.x has been
the actively maintained release series since June 2020.

Continuing to build and test against Spark 2 adds maintenance overhead
(cross-builds, dependency constraints, testing) for a runtime that upstream
no longer supports.

Proposal:
- Mark the Spark 2 connector as deprecated in the next release, with a note
in the release notes.
- Remove Spark 2 build/test targets in a subsequent release.

Please share any concerns, particularly if you're aware of users still on
Spark 2 who would be affected.

Thanks,
Zoltan