chore: migrate java-firestore to monorepo #16629

Job	Run time
gapic-bom	1m 52s
gitignore_correctness	1m 8s
generate-readme	58s
owlbot-py	1m 3s
group_id_check_for_maps_libraries	6m 56s
root-pom	1m 59s
owlbot-yaml	1m 7s
package_name_check	1m 11s
	16m 14s

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