Update deprecated GoogleCredential usage

[pavlospt]

Hey there!

Since we are using this great plugin in our app, I wanted to contribute something back :)

This PR updates the deprecated usage of GoogleCredential class.

I have tested it with the command you added on CONTRIBUTING.md (./gradlew tPublishBundle).

Looking forward to your feedback!

[pavlospt]

I believe the CI failure is irrelevant to the PR!

[SUPERCILEX]

Thanks for taking initiative, but unfortunately, this change isn't desired:

• The CI failure is real and caused by an incompatible Guava dependency. Typically things will still work if you just exclude the com.google.guava module from the google api libs.
• The new Google auth doesn't support PKCS12 keys which means we can't switch over to the new stuff until a breaking v3 version of GPP is released. I want to wait a bit longer before doing that.
  • PS: your code compiles because serviceAccountCredentials is a property of PlayPublisherExtension.Config, but it won't work at runtime.

[pavlospt]

Cool! Thanks for letting me know!