ci: replace macos-11 runners with macos-14

Since GitHub does not support `macos-11` runners anymore, jobs using
these runners just hang. Let's replace them with the most actual
`macos-14` runners. Now we test the project against 12 and 14 versions:
we could fill the gap, but let's not create new runners without actual
need.

Closes #402

Author: drewdzzz

.github/workflows/testing.yml

@@ -136,7 +136,7 @@ jobs:
           - '1.20'
           - 'stable'
         runs-on:
-          - macos-11
+          - macos-14
           - macos-12
         tarantool:
           - brew