feat(ui5-date/time*): introduce translatable fallback placeholder

[hinzzx]

Previously, when no placeholder was provided in our Date/Time components, the default placeholder simply displayed the formatPattern string (e.g., MMM d, y, dd/MM/yyyy - dd/MM/yyyy).

With this change, if no placeholder is set, the default placeholder now displays a translatable example value in the correct formatPattern/displayFormat— using the last value of the relevant period (such as the last day of the year, last hour of the day, etc.).

Examples:

e.g. 23:59:59 e.g. Dec 31, 2025

Before

After

Fixes: #11749