chore: migrate java-firestore to monorepo #16625

Job	Run time
owlbot-py	1m 7s
root-pom	1m 40s
group_id_check_for_maps_libraries	6m 45s
gapic-bom	1m 37s
generate-readme	1m 6s
owlbot-yaml	58s
package_name_check	1m 5s
gitignore_correctness	59s
	15m 17s

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