ci: add macOS 12

At some point `macos-11.0` label was replaced by `macos-11`, see [1],
[2], [3]. Moreover, `macos-12.0` does not work at all, so let's use the
similar named `macos-11` and `macos-12` (as current documentation anyway
suggests).

[1]: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idruns-on
[2]: actions/runner-images#3427
[3]: actions/runner-images#3633

.github/workflows/testing.yml

@@ -121,7 +121,8 @@ jobs:
       fail-fast: false
       matrix:
         runs-on:
-          - macos-11.0
+          - macos-11
+          - macos-12
         tarantool:
           - brew
           - 1.10.7