Patch greatuk 1631 bump wagtail 6.3

[davidu1975]

What

Upgrade Wagtail to 6.3

Why

Wagtail 6.3 is latest LTS

Tick or delete as appropriate:

Workflow

• Ticket exists in Jira https://uktrade.atlassian.net/browse/TICKET_ID_HERE
• Jira ticket has the correct status.
• A clear/description pull request messaged added.

Reviewing help

• Explains how to test locally, including how to set up appropriate data
• Includes screenshot(s) - ideally before and after, but at least after
• Documentation has been updated as necessary
• Where a PR contains code changes developed or maintained by multiple squads a representative from those squads should review the PR.

Housekeeping

• Added all new environment variables to Vault.
• Cleaned up old feature flags
• Upgraded any vulnerable dependencies.
• I have updated security dependencies
• Python requirements have been re-compiled.
• I have checked that my PR is using the latest package versions of: great-components, directory-constants, directory-healthcheck, directory-validators, directory-components, directory-api-client, directory-ch-client, django-staff-sso-client, directory-forms-api-client, directory-sso-api-client, sigauth

Security

• Frontend assets have been re-compiled
• Checked for potential security vulnerabilities
• Ensured any sensitive data is handled appropriately

Performance

• Evaluated the performance impact of the changes
• Ensured that changes do not negatively affect application scalability.

Merging

• This PR can be merged by reviewers. (If unticked, please leave for the author to merge)