#1156 ci: Remove macOS-11 builds, add macOS-13.

The macOS 11 build runners are being deprecated and are intended
to be removed by the middle of 2024:

actions/runner-images#9255

Additionally, macOS-12 is to be deprecated in Q3 and removed in Q4,
so prepare for that now by adding a macOS-13 build.

.github/workflows/ci.yml

@@ -565,11 +565,9 @@ jobs:
       fail-fast: false
       matrix:
         os: 
-          - { version: macOS-11, xcode: '11.7' }
-          - { version: macOS-11, xcode: '12.4' }
-          - { version: macOS-11, xcode: '13.0' }
           - { version: macOS-12, xcode: '13.1' }
           - { version: macOS-12, xcode: '14.0' }
+          - { version: macOS-13, xcode: '14.3.1' }
           - { version: macOS-14, xcode: '15.2' }
 
     env: