[8504] update wagtail to 6.3.x LTS

[goapunk]

Describe your changes Update wagtail to the latest lts release. Not a lot had to be changed, so this is quite straightforward.

Tasks

• [x] PR name contains story or task reference
• [ ] Steps to recreate and test the changes
• [ ] Documentation (docs and inline)
• [ ] Tests (including n+1 and django_assert_num_queries where applicable)
• [x] Changelog