chore: migrate java-firestore to monorepo #16643

Job	Run time
root-pom	1m 52s
gapic-bom	1m 47s
generate-readme	56s
owlbot-yaml	56s
group_id_check_for_maps_libraries	7m 6s
owlbot-py	1m 7s
package_name_check	59s
gitignore_correctness	1m 14s
	15m 57s

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