Use macos-latest for CI builds

[ikhoon]

Motivation:

macos-12 is no longer supported in GitHub Actions.

• https://github.com/line/armeria/actions/runs/12177357989?pr=5941
• [macOS] The macOS 12 Actions runner image will begin deprecation on 10/7/24 and will be fully unsupported by 12/3/24 for GitHub and by 01/13/25 for ADO actions/runner-images#10721

Modifications:

• Use macos-latest instead.

Result:

Revive macOS CI

[minwoox]

👍 👍 👍

[github-actions]

🔍 Build Scan® (commit: 8527527)

Job name	Status	Build Scan®
build-ubicloud-standard-8-jdk-8	✅	https://ge.armeria.dev/s/skwjhxo6ci6b4
build-ubicloud-standard-8-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/x756zv2r3bjh2
build-ubicloud-standard-8-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/cokilnymo62fc
build-ubicloud-standard-8-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/as7ustyk7f5gc
build-ubicloud-standard-8-jdk-17-leak	✅	https://ge.armeria.dev/s/uulirpwuoesvs
build-ubicloud-standard-8-jdk-11	❌ (failure)	https://ge.armeria.dev/s/de7r435brytdc
build-macos-latest-jdk-21	✅	https://ge.armeria.dev/s/pizjg5a5jxaqa