chore: migrate java-pubsub to monorepo #16920

Job	Run time
root-pom	1m 42s
owlbot-py	1m 6s
generate-readme	1m 2s
gapic-bom	1m 43s
owlbot-yaml	57s
gitignore_correctness	59s
package_name_check	59s
group_id_check_for_maps_libraries	7m 31s
	15m 59s

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