Deprecate first edition in dbplyr_fallback

[hadley]

Needs minor version bump; must happen after April 1, 2021.

[mgirlich]

@hadley I'm not quite sure how the deprecation should look like. Can you take care of that so that we can include into the next release?

[hadley]

Notes to self:

• Details of overall process in https://dbplyr.tidyverse.org/articles/backend-2.html
• So probably needs some warning in dbplyr_fallback() if the edition is 1, encouraging the user to upgrade the package. Not sure how to tell them which package to upgrade. Maybe inspect con?

[hadley]

@mgirlich it's likely to be at least 10 days before I can get to this