CLN: replace %s syntax with .format in pandas.util
Progress towards #16130

- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Replaced `%s` syntax with `.format` in pandas.util.  Additionally, made some of the existing positional `.format` code more explicit.  Left the `Substitution` class in _decorators.py as-is, since it appears to impact docstrings across the entire codebase.

