Use macOS 12

GitHub is deprecating (and eventually removing) its macOS 10.15 runners.
See actions/runner-images#5583. We might as
well take the opportunity to upgrade the macOS version we are building for.

Author: RyanGlScott

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-20.04, ubuntu-18.04, macos-10.15, windows-2019]
+        os: [ubuntu-20.04, ubuntu-18.04, macos-12, windows-2019]
         solver: [abc, boolector, cvc4, cvc5, yices, z3]
     steps:
       - uses: actions/checkout@v2
@@ -93,7 +93,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-20.04, ubuntu-18.04, macos-10.15, windows-2019]
+        os: [ubuntu-20.04, ubuntu-18.04, macos-12, windows-2019]
     steps:
 
       - uses: actions/download-artifact@v2