Update UI text in Google Cloud integration

Reopens !147463 (closed)

What does this MR do and why?

Update UI text in Google Cloud integration.

• Move permissions needed for GAR integration from docs into UI
• Use “complete the following fields” in GAR integration per https://docs.gitlab.com/ee/development/documentation/styleguide/index.html#documenting-multiple-fields-at-once
• Remove bold from IAM integration intro paragraph for consistency

EE: true

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Simulate a SAAS instance in local GDK

2. In rails console enable the experiment

   Feature.enable(:google_cloud_support_feature_flag)

3. Visit Project > Settings > Integrations > Google Cloud IAM and complete the fields with random values.

4. Visit Project > Settings > Integrations > Google Artifact Management.

5. Confirm the changes.