Add warning for users who already use ClassicLink

[elblivion]

Glad I noticed this on an otherwise diff-free plan, or it could have caused us some real grief!

[catsby]

Hey @elblivion thanks for writing in! That must have been an unpleasant find, I apologize. I don't believe that's how Terraform should behave, so I changed it in #4933 so now this plan should not happen in the future if this situation were to repeat. It's now computed, so TF would just go about it's business and change nothing (and plan nothing with this regard)

Thanks for brining this to our attention!

[elblivion]

Thanks @catsby !

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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