Reconsider usage of gradle release plugin

Currently, we use the gradle release plugin in a way probably not matching plugins own expectations. Some of this was discussed in [1]

After release branch was cut, we call [2]
```

./gradlew release

```

Apart from doing some validations, this creates two commits changing version property
- sets version in gradle.properties to '${RELEASE}-RC${RC_NUM}' (Commit_1)
- sets version in gradle.properties to back to '${RELEASE}-SNAPSHOT' (Commit_2)

Commit_1 will also be tagged as (tag: v${RELEASE}-RC${RC_NUM})

Afterwards, we continue with 'Commit_2' in testing, bundling and publishing. I.e. looking into source distribution published, this is not the one tagged, but its successor. This is probably suboptimal.

The release plugins expectations would probably more along the lines to actually increment next version (either patch, minor or even major) and release on that Commit_1.

Based on my current understanding, it seems easier to either
* drop usage of gradle release plugin and just fall back to a plain 'exec git tag'
* use a beam-release task which depends on gradle release checks, but does no version changes nor commits

The former has the drawback to also drop the checks done by release plugin, e.g.
* checkCommitNeeded
* checkUpdateNeeded
* checkSnapshotDependencies
* runBuildTasks
* createReleaseTag

which might be still valuable.

[1] [https://lists.apache.org/thread.html/205472bdaf3c2c5876533750d417c19b0d1078131a3dc04916082ce8@%3Cdev.beam.apache.org%3E](https://lists.apache.org/thread.html/205472bdaf3c2c5876533750d417c19b0d1078131a3dc04916082ce8@%3Cdev.beam.apache.org%3E)
[2] [https://github.com/apache/beam/blob/master/release/src/main/scripts/build_release_candidate.sh#L92-L94](https://github.com/apache/beam/blob/master/release/src/main/scripts/build_release_candidate.sh#L92-L94)

Imported from Jira [BEAM-6798](https://issues.apache.org/jira/browse/BEAM-6798). Original Jira may contain additional context.
Reported by: michel.