Tag bigtable-0.31.1 release.

Release 1.6.0 (googleapis#6469)

Release 1.6.1 (googleapis#6472)

Release api_core 1.5.2 (googleapis#6471)

* Release 1.5.2

Release bigquery 1.7.0 (googleapis#6399)

* Release 1.7.0

* Fix whitespace

* Add new keyword properties to changelog.

* markdown corrections.

Release securitycenter 0.1.0 (googleapis#6370)

* Release 0.1.0

Tag 'api_core-1.5.1' release.

Release datastore 1.7.1 (googleapis#6325)

Release 0.9.0 (googleapis#6268)

Release 1.6.0a1 (googleapis#6273)