[PM-31133] Add desktop-migration-milestone-4 flag

[Hinton]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-31133

📔 Objective

Add desktop-migration-milestone-4 flag

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation team

🦮 Reviewer guidelines

• 👍 (:+1:) or similar for great changes
• 📝 (:memo:) or ℹ️ (:information_source:) for notes or general info
• ❓ (:question:) for questions
• 🤔 (:thinking:) or 💭 (:thought_balloon:) for more open inquiry that's not quite a confirmed issue and could potentially benefit from discussion
• 🎨 (:art:) for suggestions / improvements
• ❌ (:x:) or ⚠️ (:warning:) for more significant problems or concerns needing attention
• 🌱 (:seedling:) or ♻️ (:recycle:) for future improvements or indications of technical debt
• ⛏ (:pick:) for minor or nitpick changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.00%. Comparing base (75a8570) to head (6c52c00).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6881   +/-   ##
=======================================
  Coverage   56.00%   56.00%           
=======================================
  Files        1966     1966           
  Lines       86875    86875           
  Branches     7737     7737           
=======================================
  Hits        48650    48650           
  Misses      36426    36426           
  Partials     1799     1799           

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