feat: Update to Wagtail 6.3.1 LTS

[kesara]

Fixes #516

This requires to run following SQL command to create an index before the deployment¹:

CREATE INDEX taggit_tagg_content_8fc721_idx ON taggit_taggeditem (content_type_id, object_id);

NOTE:
Check if the index for taggit exists first.

SELECT 
    indexname AS index_name,
    indexdef AS index_definition
FROM 
    pg_indexes
WHERE 
    tablename = 'taggit_taggeditem';

Footnotes

1. https://github.com/jazzband/django-taggit/issues/882#issuecomment-1909688346 ↩