Tag 2.8.0 release

[Gradle Release Plugin] - creating tag: 'v2.8.0-RC1'.

[Gradle Release Plugin] - creating tag: 'v2.7.0-RC3'.

[Gradle Release Plugin] - creating tag: 'v2.7.0-RC3'.

[Gradle Release Plugin] - creating tag: 'v2.7.0-RC2'.

[Gradle Release Plugin] - creating tag: 'v2.7.0-RC1'.

Apache Beam 2.6.0 Release

[Gradle Release Plugin] - creating tag: 'v2.6.0-RC2'.

[Gradle Release Plugin] - creating tag: 'v2.6.0-RC1'.

2.5.0 release