Upgrade notice to inform users of major change prior to 3.0.0 update

[mukeshpanchal27]

Summary

Fixes #655

Checklist

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[felixarntz]

Please take a look at #656 (comment) where we can coordinate the next steps on how to proceed with this PR and #1011.

[felixarntz]

@mukeshpanchal27 Thanks, this looks good to me, pending @westonruter's suggestion.

I'm putting a "request changes" similar to #1011, just until we decide how to proceed with this. See my above comment.

[felixarntz]

@mukeshpanchal27 LGTM, though we should probably merge #1011 first.