Test plugin compatibility across supported SonarQube versions

[ThomasPerkins1123]

What

Adds automated compatibility testing across the range of SonarQube versions
the plugin claims to support in the Marketplace properties file, so those
version ranges are backed by real end-to-end evidence rather than assumption.

Changes

• Parameterize the integration test — the SonarQube Docker image is now
  configurable via -Dsonarqube.image=<tag> (or the SONARQUBE_IMAGE env
  var),
  defaulting to 9.9-community. Previously the image was hardcoded.
• CI compatibility matrix — a new compatibility-matrix job runs the
  integration test against the oldest tested version and both ends of the
  supported Community Build range (9.9, 25.1, 26.7), and raises
  vm.max_map_count so newer SonarQube's Elasticsearch can start.
• Token-based scanner authentication — analysis now authenticates with a
  generated user token instead of username/password, which newer SonarQube
  versions reject. The token is passed via both sonar.token and
  sonar.login
  for compatibility across the full version range.
• Bump Testcontainers 1.19.3 → 1.20.4 for compatibility with modern Docker
  daemons.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud