chore(main): release 2.31.0 #4543

Job	Run time
build(21) except self-service clients	10m 44s
build(8) except for gapic-generator-java	3m 39s
build(8) for gapic-generator-java	9m 19s
build (11)	11m 49s
build (17)	12m 20s
compatibility	1m 24s
lint	19s
gapic-generator-java-bom	1m 31s
showcase (11)	4m 22s
showcase (17)	4m 49s
showcase-clirr	2m 55s
showcase-native	9m 31s
unmanaged_dependency_check	3m 56s
	1h 16m 38s

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