Wagtail 6.3, Django 5.1, Python 3.13

[zerolab]

This PR does what it says on the tin

[helenb]

Thanks @zerolab. There had been a discussion with Sam T about leaving wagtail upgrades while there's a big piece of work going on on the site.

[zerolab]

These changes should not conflict with the other work, really.
And it would be a good one to battle test while the other features are being tested too.

I am not too fussed. It also makes sense to me to push the improvements sooner, rather than later

[SharmaineLim]

I had a poke around and things mostly look okay. All internal links were broken initially until I re-published one of the pages (the homepage specifically).

[SharmaineLim]

^ There was some new code in main that used the method assertQuerysetEqual which is removed in Django 5.