chore: migrate java-spanner to monorepo #16658

Job	Run time
group_id_check_for_maps_libraries	6m 36s
package_name_check	24s
gitignore_correctness	56s
gapic-bom	1m 8s
owlbot-yaml	46s
generate-readme	1m 19s
owlbot-py	1m 5s
root-pom	1m 39s
	13m 53s

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