Merge pull request apache#14667:[BEAM-12343] GroupByKeyTest for chang…

…ing WindowFn from GlobalWindow after GBK

Version 2.29.0

Few small updates to the release guide

Tag for 2.28.0 release.

[Gradle Release Plugin] - creating tag: 'v2.28.0-RC1'.

Beam release 2.27.0

[Gradle Release Plugin] - creating tag: 'v2.27.0-RC4'.

[Gradle Release Plugin] - creating tag: 'v2.27.0-RC3'.

[Gradle Release Plugin] - creating tag: 'v2.27.0-RC1'.