ci: use macos 11 in virtual environment

ci: use macos 11 in virtual environment

GitHub Actions was recently enabled in commit ce1b5612ce99 ("ci: add CI
via GitHub Actions").

The commit has a AuthorDate of 2020.

The commit mentions that it uses macOS 10.15 rather than 11.0 because
11.0 is only private preview.

This was true in 2020, but looking at:
https://docs.github.com/en/actions/reference/specifications-for-github-hosted-runners

macos-11 is no longer marked as private preview, so let's use it in the
virtual environment.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Link: https://lore.kernel.org/r/20211214111756.52968-2-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>