ci: run on macOS-11 because there no more github runners for macOS 10

Jobs are no longer being picked up:
https://github.com/overhangio/tutor/actions/runs/5259868936/jobs/9515592636

Author: regisb

.github/workflows/release.yml

@@ -25,7 +25,7 @@ jobs:
           - os: ubuntu-20.04
             locale: C.UTF-8
           # https://endoflife.date/macos
-          - os: macos-10.15
+          - os: macos-11
             locale: en_US.UTF-8
     env:
       LC_ALL: ${{ matrix.locale }}