firebase13: bump minimum macOS to 11.0 and watchOS to 8.0

[ncooke3]

Update deployment targets across Package.swift, podspecs, project build settings, and CI scripts from macOS 10.15 to macOS 11.0 and watchOS 7.0 to watchOS 8.0.

Clean up legacy macOS < 11 MIME type availability handling in StorageUtils.swift and add a breaking change entry to FirebaseCore/CHANGELOG.md.

Quickstart PR: todo

Fix #16260

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