Upgrade codecov action to v5

[KulaginVladimir]

Proposed changes

Upgrades codecov action to v5 in the CI workflow.

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.31%. Comparing base (678eb1a) to head (78071b4).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #942   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          60       60           
  Lines        2905     2905           
=======================================
  Hits         2885     2885           
  Misses         20       20           

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🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.