Implement filtering by severity and release channel

This also involves a refactoring of the way reports are encoded: rather than using plaintext error messages, use readable codes and structured metadata for errors and updates to make them easier to process.

️🧪 Test rendering here: https://infrastructure.pages.apertis.org/-/dashboard/-/jobs/1376068/artifacts/public/index.html

Task: https://phabricator.apertis.org/T7889

.gitlab-ci.yml

@@ -266,6 +266,7 @@ packaging-updates:
   before_script:
     - apt update && apt install -y --no-install-recommends
         python3-debian
+        python3-gitlab
         python3-yaml
   script:
     - ./bin/packaging-updates
@@ -287,6 +288,7 @@ packaging-updates-upstream-linux:
         ca-certificates
         git
         python3-debian
+        python3-gitlab
         python3-yaml
   script:
     - ./bin/packaging-updates-upstream-linux