chore: migrate java-pubsub to monorepo #16650

Job	Run time
group_id_check_for_maps_libraries	6m 47s
gapic-bom	1m 45s
root-pom	1m 45s
owlbot-yaml	1m 2s
gitignore_correctness	55s
owlbot-py	1m 4s
generate-readme	1m 10s
package_name_check	54s
	15m 22s

Provide feedback