PERF: pd.to_datetime, unit='s' much slower for float64 than for int64
- [x] closes #20445
- [x] tests passed
- [x] new benchmarks added
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

As per discussion in #20445 this PR addresses performance of `to_datetime` on a uniform type array of floats. The aim is to get a speed-up by implementing `astype`-ing, and avoid looping, for floats.