chore: migrate java-pubsub to monorepo #16649

Job	Run time
group_id_check_for_maps_libraries	6m 58s
generate-readme	1m 7s
gitignore_correctness	1m 5s
owlbot-py	1m 15s
gapic-bom	1m 31s
root-pom	1m 45s
owlbot-yaml	54s
package_name_check	58s
	15m 33s

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