test: change GitHub Actions macOS platform to macOS 12

I am seeing some documentation build failures on macOS 11. See if the
issue is fixed in macOS 12.

https://github.com/vincentkfu/fio/actions/runs/3395509649/jobs/5645674083

This change ultimately did not resolve the issue but we might as well
switch to macOS 12 since it has been out for a year already.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>

Author: vincentkfu

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
           os: ubuntu-22.04
           cc: clang
         - build: macos
-          os: macos-11
+          os: macos-12
         - build: linux-i686-gcc
           os: ubuntu-22.04
           arch: i686