ENH: Support strings containing '%' in add_prefix/add_suffix (#17151)
 - [X] closes #17151
 - [X] tests added / passed
 - [X] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [X] whatsnew entry

Updated the `DataFrame.add_prefix` and `DataFrame.add_suffix` methods to use the new style `.format` syntax to perform the string formatting.  These previously used the old style string formatting, which raises when the prefix/suffix to add contains the '%' character.