Upgrade to AdminLTE v4

AdminLTE is currently used in its v3 version, based on Bootstrap 4.
Wicket components are instead working with Bootstrap 5, hence a few glitches can be seen (modal windows do not close correctly, drop downs not showing...).

AdminLTE v4 (not yet released at present) will be instead based on Bootstrap 5 so this will align everything - see https://github.com/ColorlibHQ/AdminLTE