chore: add monorepo migration script #15933

Job	Run time
owlbot-yaml	38s
gitignore_correctness	37s
root-pom	1m 22s
owlbot-py	46s
group_id_check_for_maps_libraries	6m 8s
generate-readme	44s
gapic-bom	1m 51s
package_name_check	38s
	12m 44s

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