Update grants.md for Redshift roles and groups

[psaintjust]

Added instructions for using grants with Redshift roles and groups.

What are you changing in this pull request and why?

Added instructions for using grants with Redshift roles and groups since it requires a Redshift specific syntax.

Checklist

• Review the Content style guide so my content adheres to these guidelines.
• For docs versioning, review how to version a whole page and version a block of content.
• Add a checklist item for anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

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Hello!👋 Thanks for contributing to the dbt product documentation and opening this pull request! ✨
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Name	Status	Preview	Comments	Updated (UTC)
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[mirnawong1]

Thanks for opening up this pr @psaintjust ! I'll take a look at this for you!

[mirnawong1]

raised this question in internal slack for additional insight

[mirnawong1]

hey @psaintjust ! this is a great pr and i appreciate you opening it up! after a conversation with @dbeatty10 and the internal team, we want to hold off on documenting how to use roles and groups in Redshift until dbt-redshift #415 and dbt-redshift #626 are resolved.

This is mostly because:

• If there’s a code change, the syntax could break
• The below syntax isn’t the intended behavior as of right now, That this syntax doesn’t raise an error is completely accidental:
  {{ config(grants = {‘select’: [‘ROLE role_analyst’]}) }}
• Unlike Redshift users, grants done in this manner will remain forever unless they are revoked manually. This is exactly the opposite of our intention with the grants feature.

so i’m going to close this pr out, however, i’ve added your suggestions to this issue here, which is being blocked by development. let me know if you have any questions at all and i really appreciate your thorough contribution!