Fix for Gradle 7 Compatibility

[abeytom07]

The gw publish throws the following error while used with gradle 7.0.2.

Type 'org.gradle.api.publish.maven.tasks.PublishToMavenRepository' property 'credentials.password' is missing an input or output annotation.    
Reason: A property without annotation isn't considered during up-to-date checking.
Possible solutions:
      1. Add an input or output annotation.
      2. Mark it as @Internal.

Fixes #46

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[abeytom07]

@googlebot I signed it!

[abeytom07]

Any Updates?

[lindsayismith]

/gcbrun

[lindsayismith]

Thanks for the PR, much appreciated!