chore: note and prevent dependabot from updating FluentAssertions >=8 (#199)

This leaves a comment in `.csproj`s and adds an explicit exception to
the dependabot config not to upgrade FluentAssertions to >=8.0.0, which
changes the package's license to paid for commercial uses.

This also pins Ubuntu to 22.04, since `mono`, which is a dependency for
our tests, is not available on latest 24. See:
actions/runner-images#10636

Ref: dequelabs/bass-team#138

Author: scottmries

.github/workflows/build-and-test.yml

@@ -16,7 +16,7 @@ on:
 jobs:
   build-and-test:
     name: Build and test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     timeout-minutes: 20
     steps:
       - name: Checkout repo