PERF: Improve performance in to_csv with date format for index

[phofl]

• closes BUG: Performance regression to_csv when formatting datatime index #39413
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

Before:

%timeit df.to_csv(date_format='%Y-%m-%d %H:%M:%S')
1.05 s ± 11.5 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

afterwards:

%timeit df.to_csv(date_format='%Y-%m-%d %H:%M:%S')
392 ms ± 4.31 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

[jbrockmendel]

LGTM

[jreback]

thanks @phofl wouldn't object to addtitional benchmarks as specified in the OP

[phofl]

We have some of them already, will check and add as necessary