deps: update dependency com.google.api-client:google-api-client-bom to v2.6.0 #6930

Job	Run time
build (11)	3m 8s
build (17)	3m 2s
build(21) except self-service clients	3m 19s
build(8) except for gapic-generator-java	3m 41s
build(8) for gapic-generator-java	1m 39s
compatibility	1m 38s
lint	38s
showcase (11)	1m 31s
showcase (17)	1m 40s
dependency_analyzer_unit_test	45s
gapic-generator-java-bom	1m 28s
showcase-clirr	3m 35s
showcase-native	1m 23s
unmanaged_dependency_check	4m 16s
	31m 43s

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