go_resource deprecation comments

[cho-m]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Continuing discussion at #16971 (comment) and following up on related deprecation.

All usage has been removed from Homebrew/core.

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Open to recommendations for better message.

I just simplified it to say Use Go modules instead under the assumption that Go modules is known and well documented. It isn't exactly a "replacement" in Homebrew but an upstream build change.

[MikeMcQuaid]

Yeh, this language makes more sense, thanks again!