Handle single quotes in PostgreSQL default column values

PostgreSQL escapes single quotes by using an additional single quote. When Rails queries the column information, PostgreSQL returns the default values with the escaped single quotes.

`#extract_value_from_default` now converts these to one single quote each.

Fixes #10881.

(This is my first time digging into the ActiveRecord source, so I took a stab in the dark with where to include the tests for this; if anyone has any input it'd be appreciated. Thanks!)