[chore] Combine GCP attributes

[joaopgrassi]

Follow up from #849 (comment)

Now all GCP attributes are in a single file in the registry gcp.yaml|md

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

CC @trisch-me

[joaopgrassi]

I will delete the old area: labels once this is merged. A new one will be automatically created for area:gcp.

[trisch-me]

great, thank you!