Fix User Date of Birth Validation and Processing

[smg6511]

What does it do?

Replace usage of strtotime with DateTimeImmutable::createFromFormat for better format compatibility.

Why is it needed?

Certain date formats, such as m-d-Y, are incompatible with strtotime and will cause the value of input fields where it's currently used (such as in a User's Profile) to fail.

How to test

Set the system setting manager_date_format to m-d-Y (or any other format where the month and day would be programmatically ambiguous), clear caches, and verify that a User's DOB can be added/updated as expected. Note that there are two distinct places to check this: under Manage/Users/[User] and via the logged in User's Profile page (at /manager/?a=security/profile)

Related issue(s)/PR(s)

n/a

[theboxer]

It'd be slightly better for the review process if changes unrelated to the issue itself would be in separate commits (like reformatting etc.), otherwise it's hard to spot what actually changed.