DEPR: Deprecate non-ISO date string formats in DatetimeIndex.loc

[raghavendranath]

• closes BUG: .loc with DateTimeIndex allows day first string for stop in slice, but not start #58302
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Before:

ser.loc["1/10/2024":"1/14/2024"]      # Returns Jan 10-14
ser.loc["1/10/2024":"14/1/2024"]      # Also returns Jan 10-14
ser.loc["10/1/2024":"14/1/2024"]      # Returns empty

After:

ser.loc["1/10/2024":"1/14/2024"]      # Still returns Jan 10-14 + FutureWarning
ser.loc["2024-01-10":"2024-01-14"]    # Use ISO format instead

There's no way to know if "1/10/2024" is being parsed as MM/DD or DD/MM until you run it. This deprecation pushes users toward ISO format to avoid the confusion.