chore: Drop Scala 2.12 #1080

Job	Run time
Checks	3m 47s
Compile Benchmarks	4m 21s
Compile and test (2.13)	8m 35s
Docs	9m 9s
Gradle tests	6m 57s
Maven tests	5m 52s
Compile and test (3.3)	8m 59s
sbt scripted tests (gen-scala-server, 2.13)	16m 35s
sbt scripted tests (gen-java, 2.13)	8m 56s
sbt scripted tests (scala-2_13, 2.13)	6m 46s
sbt scripted tests (scala3, 3.3)	8m 53s
	1h 28m 50s

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