BUG: Align Series.str.zfill() with str.zfill()
- [ ] closes #20868
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Using standard library `zfill` method directly to handle specific characters issue
@atulagrwl : Good start!  Need a `whatsnew` entry in `0.24.0` as well. Added the validation test in last commit and added whatsnew entry too. can you rebase closing as stale. if you'd like to continue, pls ping.