Move CI runners away from macOS-10.15 because of sunsetting by Github.

There's a scheduled removal of the image by 30 Aug 2022, with intermittent
failing of those runners to raise awareness to that effect. We need to
move our CI runners to at least `macOS-11` to preempt any failures.

Fixes diffblue#7027.

Author: NlightNFotis

.github/workflows/pull-request-checks.yaml

@@ -382,8 +382,8 @@ jobs:
       - name: Run tests
         run: cd build; ctest . -V -L THOROUGH -j2
 
-  check-macos-10_15-make-clang:
-    runs-on: macos-10.15
+  check-macos-11-make-clang:
+    runs-on: macos-11
     steps:
       - uses: actions/checkout@v2
         with:

.github/workflows/release-packages.yaml

@@ -144,7 +144,7 @@ jobs:
 
 
   homebrew-pr:
-    runs-on: macos-10.15
+    runs-on: macos-11
     steps:
       - name: Get release tag name
         # The GITHUB_REF we get has refs/tags/ in front of the tag name so we