ENH: Add ignore_index to sort_index
- [x] closes #30114
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


@jreback 
I just looked back at #30114 and found out that i overlooked to add `ignore_index` to `sort_index`, and this might be the reason this issue is still left open. I added this to `sort_index` to close this issue.