chore: migrate java-spanner to monorepo #16659

Job	Run time
package_name_check	1m 2s
owlbot-py	1m 2s
root-pom	1m 40s
owlbot-yaml	55s
group_id_check_for_maps_libraries	6m 53s
gapic-bom	1m 38s
generate-readme	1m 3s
gitignore_correctness	53s
	15m 6s

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