Fix ad-hoc filter support for Django 5

[dases]

Ad-hoc filters (ie, those not declared in list_filter) is broken when using Django 5+.

Such filtering is supported when using Django 4x (also, it's supported by current Django 5+ ModelAdmin).

The issue is that we are passing around a QueryDict of query params in IndexView, and in some places calling .items() on it, which has a flattening affect on values.

This means that, for an undeclared query param:

• the get_filters method returns only the last value for a multi-value query, and
• the get_queryset passes a dict[str, str] to build_q_object_from_lookup_parameters Django util, which treats the value part of .items() as a list.

The latter means that a string value gets treated as a list that is OR'd:
E.g. given a query param: location=Hobart, we get a queryset with WHERE location='H' OR location='o' OR location='b'...

The tests that accompanied support for multi-valued query params did not capture this because the filter being tested is declared in list_filter. wagtail/wagtail#10242