ci: Bump macOS image to big-sur-xcode-12.5
ci: Bump macOS image to big-sur-xcode-12.5

This also removes the "brew update" added in commit
b7381552cd4f965c45f1560d9cfc2fb09dbfcc1d.

https://github.com/bitcoin/bitcoin/pull/22122 (1/1)

ElementsProject/elements,ElementsProject/elements,ElementsProject/elements,ElementsProject/elements,ElementsProject/elements,ElementsProject/elements