Remove upper bound from wagtail dependency

[gasman]

Since wagtail-factories is now a testing dependency of Wagtail itself, having an upper bound of <5.0 is blocking us from testing the now-in-development Wagtail 5.0.

There shouldn't be any risk in doing this, since if Wagtail makes a breaking change in future that breaks wagtail-factories, we (as Wagtail core devs) will need to fix wagtail-factories in order to proceed :-)

[zerolab]

@jams2 can we get this in? or perhaps #70 with the upper bound removed?

[zerolab]

Superceded by #74