chore: add configuration for protobuf-4.x-rc release #15559

Job	Run time
root-pom	1m 30s
gapic-bom	1m 31s
owlbot-py	1m 3s
owlbot-yaml	52s
generate-readme	1m 1s
gitignore_correctness	55s
package_name_check	53s
group_id_check_for_maps_libraries	6m 31s
	14m 16s

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