build: add monorepo migration script #15934

Job	Run time
owlbot-py	46s
generate-readme	48s
package_name_check	37s
gapic-bom	1m 15s
root-pom	1m 22s
owlbot-yaml	37s
group_id_check_for_maps_libraries	6m 4s
gitignore_correctness	36s
	12m 5s

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