User Retention: Improve default last login field

[rak-phillip]

The default last login form in the User Retention admin form requires that an admin enters the value in milliseconds. This isn't a very friendly design for several reasons:

1. It requires admins to use an external tool to convert a given date to milliseconds, as it's highly unlikely that one would know the desired date value in milliseconds
2. It can be easy to interpret any millisecond representation as a valid value. For example, an admin would be surprised to find that fractional seconds will not produce the expected value
3. It's difficult to confirm that the millisecond value is what an admin would expect, 2024-07-11T12:42:48-07:00 is much easier to understand than 1720726968000

Proposed solution

Replace the text input for default last login with a well-formatted date input. We can consider using input type of date/datetime-local or implementing a JavaScript date picker.

Screenshots

[pmatseykanets]

Is there any chance this can be done in v2.9.next1?