chore(main): release 1.6.0 #767

Job	Run time
javadoc-with-doclet (java-bigtable)	1m 36s
build (8, java-bigquery, test)	2m 1s
build (8, java-bigquery, lint)	1m 2s
build (8, java-bigquery, clirr)	1m 1s
build (8, java-bigquery, javadoc)	1m 21s
build (8, java-bigtable, test)	4m 50s
build (8, java-bigtable, lint)	1m 40s
build (8, java-bigtable, clirr)	2m 7s
build (8, java-bigtable, javadoc)	2m 52s
build (11, java-bigquery, test)	2m 38s
build (11, java-bigquery, lint)	1m 35s
build (11, java-bigquery, clirr)	1m 21s
build (11, java-bigquery, javadoc)	1m 38s
build (11, java-bigtable, test)	3m 28s
build (11, java-bigtable, lint)	2m 14s
build (11, java-bigtable, clirr)	2m 21s
build (11, java-bigtable, javadoc)	3m 11s
	36m 56s

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