Closes apache#3874

Tag version 2.1.0

[maven-release-plugin] copy for tag v2.1.0-RC3

[maven-release-plugin] copy for tag v2.1.0-RC2

[maven-release-plugin] copy for tag v2.1.0-RC1

Apache Beam, version 2.0.0

[maven-release-plugin] copy for tag v2.0.0-RC4

[maven-release-plugin] copy for tag v2.0.0-RC3

[maven-release-plugin] copy for tag v2.0.0-RC2

[maven-release-plugin] copy for tag v2.0.0-RC1