chore: DBTP-1700 Deprecate cross_enviroment_service_access application property

[WillGibson]

Found while working on DBTP-1700.

It is a required field, but not used for anything 😬

Just made it optional for now and will document it as deprecated in https://github.com/uktrade/platform-documentation/pull/498.

Goes with uktrade/platform-tools#780 and https://github.com/uktrade/demodjango-deploy/pull/286.

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Checklist:

Title:

• Scope included as per conventional commits
• Ticket reference included (unless it's a quick out of ticket thing)

Description:

• Link to ticket included (unless it's a quick out of ticket thing)
• Includes tests (or an explanation for why it doesn't)
• Includes any applicable changes to the documentation in this code base
• Includes link(s) to any applicable changes to the documentation in the DBT Platform Documentation (can be to a pull request)

Tasks:

• Trigger the pull request regression tests for this branch and confirm that they are passing

End to end test run.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.68%. Comparing base (e1c9d77) to head (0b78dee).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #345   +/-   ##
=======================================
  Coverage   81.68%   81.68%           
=======================================
  Files           7        7           
  Lines         868      868           
=======================================
  Hits          709      709           
  Misses        159      159           

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