There should be single source of truth for the project version

Now the Beam version is defined in two places:
* via the 'version' parameter [in gradle.properties](https://github.com/apache/beam/blob/master/gradle.properties#L27),
* programmatically in the `apply` function [in BeamModulePlugin](https://github.com/apache/beam/blob/cd653e33b342bd09c76c2bbaca12597fec5b4a2c/buildSrc/src/main/groovy/org/apache/beam/gradle/BeamModulePlugin.groovy#L494)

The latter effectively overrides the former. For someone not very familiar with the Beam build system this may result in a hard to understand and debug behavior: unexpected version gets build and/or pushed to local Maven repository, and changing version in `gradle.properties` does not have any effect.

Adding a comment with a reference to `BeamModulePlugin` to `gradle.properties` can be a good start, and in the long run it would be great to have single source of truth for the project version.

Imported from Jira [BEAM-14051](https://issues.apache.org/jira/browse/BEAM-14051). Original Jira may contain additional context.
Reported by: dmitryor.