Brakeman: return a zero status code if any warnings are found

Brakeman returns a non-zero status code when there are warnings found or
errors during the scan.

In 9fc187b, we allowed the GitHub
Action to continue on error, so that the warning/error would be uploaded
to GitHub code scanning.

However the non-zero status code still gets written to the GitHub Action
output. This led to confusion in
alphagov/asset-manager#1583 as we thought the
error meant brakeman did not complete properly, when in fact it did but
was only returning that code because a code scanning warning had been
found.

Therefore updating the brakeman configuration to report a zero status
code when a warning or error is found. This means we can remove the
previous change, as any completed run will not return a non-zero status
code. This will also stop the non-zero code causing confusion in the
GitHub Actions output.

.github/workflows/brakeman.yml

@@ -21,9 +21,9 @@ jobs:
           bundler-cache: true
 
       - name: Run Brakeman
-        continue-on-error: true
         run: |
           bundle exec brakeman . --except CheckRenderInline --quiet \
+            --no-exit-on-warn --no-exit-on-error \
             -o brakeman.json -o brakeman.sarif
 
       - name: Upload SARIF to GitHub