Bump back iOS CI runners to macos-15

[antonis]

The macos CI runners were temporarily downgraded to tackle the deprecations listed in actions/runner-images#12734 with #5079

We should revisit this and properly handle the following failures by updating the projects when possible:

• The RnDiffApp for the specific React Native version was failing to build due to missing 18.2 platform
• Expo sample failing due to missing 18.0 platform
• RN Sample failing due to missing 18.0 platform

Relevant PRs:

• chore(ci): Handle GH runner image deprecations #5079
• chore(ci): Use macos-14 for testflight workflow, native tests and legacy rn #5083
• chore(ci): Fallback to macos-14 for React Native check sentry-cocoa#6043

[linear]

RN-300 Bump iOS CI runners to macos-15