Upgrade wagtail vulnerability fix

[sebs-code]

This ticket upgrades Wagtail to address vulnerability concerns.

Workflow

• Ticket exists in Jira https://uktrade.atlassian.net/browse/GREATUK-1022
• Jira ticket has the correct status.
• A clear/description pull request messaged added.

Merging

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.78%. Comparing base (0f3bc9f) to head (0920d01).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1193   +/-   ##
========================================
  Coverage    94.78%   94.78%           
========================================
  Files           80       80           
  Lines         3546     3546           
========================================
  Hits          3361     3361           
  Misses         185      185           

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