CLN: replace %s syntax with .format in core.panel
- [x] Progress towards #16130 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Note, there's still a couple usages in this file in the `Appender` decorators, I'm planning on going through all `appender`/`shared_docs` usages in the near future to clean these up too.