When prompting, use `show_default` as the default if its a string

[thejcannon]

This PR just makes sure the default handed to .prompt is the show_default if its a string (to match the help).

fixes #2836

• [x] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [X] Add or update relevant docs, in the docs folder and in code.
• [X] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [x] Add .. versionchanged:: entries in any relevant code docs.

[thejcannon]

oh duh, this changes the default 🤦 . Gotta go deeper...

[Rowlando13]

Please rebase on main to resolve the changes.rst. With merge after #3030.