Rails One Click is not maintained anymore [ci skip]

Let's tell people that they should rely on Tokaido instead of Rails One
Click.

Fixes #14960.
Convert column name to string only once
Simplify unless conditional with OR
Get rid of conditional since column_for handles raising now

column_for will raise in case column is not found for the given table,
so there is no need to handle that here.
Simplify building options hash in rename column method for mysql
Use the reader attribute rather than the instance var when possible
Fix examples indent and improve #process docs a bit [ci skip]