Deprecate runner support for Spark 2.4 (closes #22094)

[mosche]

It is time to deprecate the Spark 2 runner in Beam, see this discussion. Databricks is also dropping support for runtimes using Spark 2 by the end of June 2022.

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[mosche]

R: @echauchot
R: @aromanenko-dev
R: @jbonofre

[aromanenko-dev]

CC: @iemejia

[echauchot]

Thanks Moritz !

[mosche]

Hmm, having seen the maven download stats, I'm a bit skeptical if Spark 2.4 should be deprecated in fact.
The numbers of the last month:

beam-runners-spark-3
2.30.0 22
2.31.0 22
2.32.0 16
2.33.0 18
2.34.0 17
2.35.0 47
2.36.0 160
2.37.0 17
2.38.0 114
2.39.0 23
All 472

beam-runners-spark
2.30.0 26
2.31.0 24
2.32.0 69
2.33.0 403
2.34.0 352
2.35.0 1543
2.36.0 50
2.37.0 19
2.38.0 420
2.39.0 86
All 5224