ci: bump checkout@v4 to v5 in GHA workflows

[josephyi]

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• I have added information to docs/ if required so people know about the feature!
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• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

• Bumps Github's checkout action from v4 to v5.

How did you test this code?

Local runs and have performed this bump on other repos.

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3 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
docs	Ignored	Preview		Oct 20, 2025 2:35pm
flagsmith-frontend-preview	Ignored	Preview		Oct 20, 2025 2:35pm
flagsmith-frontend-staging	Ignored	Preview		Oct 20, 2025 2:35pm

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.00%. Comparing base (547139a) to head (64341bd).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6154   +/-   ##
=======================================
  Coverage   98.00%   98.00%           
=======================================
  Files        1278     1278           
  Lines       44983    44983           
=======================================
  Hits        44084    44084           
  Misses        899      899           

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