MID-11078 Fix false dateTime updates due to seconds normalization in admin GUI

[kay1313]

The admin GUI date picker operates at minute precision and drops seconds/milliseconds on submit.
The existing guard in XmlGregorianCalendarModel used GregorianCalendar.equals(...), which compares full calendar state (including timezone), not just the actual time value.

This caused false differences when:

-stored value contained seconds (e.g. 09:32:12),
-GUI round-trip produced 09:32:00,
-or calendars differed only in internal representation (e.g. timezone/configuration),
-even though both represented the same effective minute.
As a result, the GUI incorrectly overwrote values and preview showed spurious changes.

Solution

Replace strict GregorianCalendar.equals(...) with comparison of normalized minute-precision timestamps using getTimeInMillis() after stripping seconds and milliseconds.

This ensures:
-no change is produced when only GUI precision differs,
-false differences caused by calendar metadata (including timezone representation) are avoided,
-real differences in actual time are still detected.
In particular:
-real timezone differences → still cause update (correct)
-fake differences from calendar equality → no longer cause update (fixed)