Merge bitcoin#21542: ci: Bump macOS VM image to the latest version

b8e7647 ci: Bump macOS VM image to the latest version (Hennadii Stepanov)

Pull request description:

  On Cirrus CI bump macOS VM from Catalina to Big Sur.

ACKs for top commit:
  fanquake:
    ACK b8e7647 - I'm always going to question bumping things for the sake of it/when there's no reasoning given. In this case, moving from building on macOS 10.x to 11.x shouldn't really lose us any coverage, and may turn up potential macOS quirks a bit earlier.

Tree-SHA512: ab2daee194683ab0553328020fd2fcb918160f466cd380c542e1a9b44f5bea3664fb40b032f1b611ee0107b0efbe278230e067316e2373c3cb0470b205dd2f9d

Author: fanquake

.cirrus.yml

@@ -157,13 +157,13 @@ task:
     FILE_ENV: "./ci/test/00_setup_env_mac.sh"
 
 task:
-  name: 'macOS 10.15 native [gui] [no depends]'
+  name: 'macOS 11 native [gui] [no depends]'
   macos_brew_addon_script:
     - brew install boost libevent berkeley-db4 qt miniupnpc ccache zeromq qrencode sqlite libtool automake pkg-config gnu-getopt
   << : *GLOBAL_TASK_TEMPLATE
   macos_instance:
     # Use latest image, but hardcode version to avoid silent upgrades (and breaks)
-    image: catalina-xcode-12.1  # https://cirrus-ci.org/guide/macOS
+    image: big-sur-xcode-12.4  # https://cirrus-ci.org/guide/macOS
   env:
     DANGER_RUN_CI_ON_HOST: "true"
     CI_USE_APT_INSTALL: "no"