ci: Use iOS18.5 since 18.6 seems to be unavailable sometimes

[itaybre]

Releases are blocked because tests fails when iOS18.6 is not found.
While Github Docs says that iOS18.6 are available, they might be in process of being deployed.

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.675%. Comparing base (1a34ddc) to head (05ae5c3).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #5944       +/-   ##
=============================================
+ Coverage   86.528%   86.675%   +0.146%     
=============================================
  Files          423       423               
  Lines        36574     36587       +13     
  Branches     15697     17332     +1635     
=============================================
+ Hits         31647     31712       +65     
+ Misses        4885      4828       -57     
- Partials        42        47        +5     

see 17 files with indirect coverage changes

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