build: add SpotBugs plugin and fix all reported findings #7155

Job	Run time
sample_operators_tests (sample-operators/webpage)	3m 53s
sample_operators_tests (sample-operators/tomcat-operator)	4m 2s
sample_operators_tests (sample-operators/leader-election)	4m 18s
sample_operators_tests (sample-operators/operations)	8m 41s
sample_operators_tests (sample-operators/mysql-schema)	3m 30s
	24m 24s

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