chore: migrate java-firestore to monorepo #16626

Job	Run time
generate-readme	1m 8s
root-pom	1m 49s
group_id_check_for_maps_libraries	7m 3s
owlbot-yaml	59s
gitignore_correctness	1m 3s
gapic-bom	1m 46s
owlbot-py	1m 9s
package_name_check	56s
	15m 53s

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