Update maven-core, maven-plugin-api to 3.9.6 #783

Job	Run time
Compile Benchmarks	3m 40s
Checks	4m 6s
Compile and test (2.12.19, 8)	5m 53s
Docs	4m 48s
Compile and test (2.12.19, 11)	5m 57s
Maven tests	4m 43s
Gradle tests	5m 29s
Compile and test (2.12.19, 17)	5m 55s
Compile and test (2.13.13, 8)	6m 28s
Compile and test (2.13.13, 11)	6m 31s
sbt scripted tests (gen-scala-server, 2.12)	10m 37s
Compile and test (2.13.13, 17)	6m 29s
sbt scripted tests (gen-java, 2.12)	5m 55s
Compile and test (3.3.3, 8)	6m 37s
Compile and test (3.3.3, 11)	6m 38s
Compile and test (3.3.3, 17)	6m 34s
	1h 36m 20s

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