BUG: Replace internal use of loc with reindex in DataFrame append
The issue here was that the DataFrame append method was using .loc, which only throws a warning now, but would eventually throw a KeyError whenever that went into effect. Just swapped out that use of .loc for .reindex.

- [x] closes #22252
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
