chore(main): release 2.40.0 #6473

Job	Run time
build (11)	12m 19s
build (17)	12m 38s
build(21) except self-service clients	11m 19s
build(8) except for gapic-generator-java	4m 6s
build(8) for gapic-generator-java	9m 31s
compatibility	1m 31s
dependency_analyzer_unit_test	15s
gapic-generator-java-bom	2m 5s
lint	27s
showcase-clirr	3m 14s
showcase-native	8m 10s
unmanaged_dependency_check	3m 44s
showcase (11)	4m 5s
showcase (17)	4m 11s
	1h 17m 35s

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