Update all the documentation related to klar, trivy and container scanning

Tasks:

• update documentation which is related to klar and container scanning. Adding anything which might be needed in relation to trivy. The following could be considered as a starting point.
  1. how to enable the v4 scanner (gcs) before it's made the default
  2. how to upgrade from the v3 (klar) to the v4 (gcs) scanner, including any configuration breaking changes
  3. how to roll-back to the v3 scanner the v4 scanner is the default
  4. include supported scanner-specific environment variables; add warning that using unsupported variables may break integration
  5. Table in doc/user/application_security/vulnerabilities/index.md
  • internal documentation updates, including:
    1. https://about.gitlab.com/handbook/engineering/development/secure/tech-docs/severity-levels/
    2. https://about.gitlab.com/handbook/engineering/development/secure/#skills
    3. Improved documentation regarding how to use vulnerability allow listing