build: add SpotBugs plugin and fix all reported findings #7154

Job	Run time
sample_operators_tests (sample-operators/mysql-schema)	4m 5s
sample_operators_tests (sample-operators/operations)	9m 45s
sample_operators_tests (sample-operators/webpage)	4m 26s
sample_operators_tests (sample-operators/leader-election)	4m 47s
sample_operators_tests (sample-operators/tomcat-operator)	4m 15s
	27m 18s

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