ci: Build on macOS 11 instead of macOS 10.15

macOS builder capacity on Cirrus CI is quite limited, and so we
can't afford to keep the old build job around after adding the
new one like we do for FreeBSD.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

.gitlab-ci.yml

@@ -424,13 +424,13 @@ x64-freebsd-12-build:
     UPDATE_COMMAND: pkg update
     INSTALL_COMMAND: pkg install -y
 
-x64-macos-1015-build:
+x64-macos-11-build:
   extends: .cirrus_build_job
   variables:
-    NAME: macos-1015
+    NAME: macos-11
     CIRRUS_VM_INSTANCE_TYPE: osx_instance
     CIRRUS_VM_IMAGE_SELECTOR: image
-    CIRRUS_VM_IMAGE_NAME: catalina-base
+    CIRRUS_VM_IMAGE_NAME: big-sur-base
     UPDATE_COMMAND: brew update
     INSTALL_COMMAND: brew install
     PATH_EXTRA: /usr/local/opt/ccache/libexec:/usr/local/opt/gettext/bin:/usr/local/opt/libpcap/bin:/usr/local/opt/libxslt/bin:/usr/local/opt/rpcgen/bin