ci: configure the protected branch #11371

Job	Run time
build(24) except self-service clients	12m 56s
build(21) except self-service clients	12m 53s
build (11)	14m 10s
build(8) except for gapic-generator-java	4m 35s
build (17)	14m 2s
build(8) for gapic-generator-java	10m 25s
lint	58s
build(8) for showcase	4m 34s
showcase-clirr	3m 32s
compatibility	1m 32s
unmanaged_dependency_check	8m 2s
gapic-generator-java-bom	1m 50s
dependency_analyzer_unit_test	15s
showcase (17)	9m 43s
showcase (21)	9m 44s
showcase (11)	9m 38s
showcase (24)	9m 37s
	2h 8m 26s

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