[x] Update to the latest release of keycloak 21 to get rid of known security issues.
[x] Add a link to the keycloak migration guide
Not updating to keycloak 22 yet, because we want to keep the version in sync with the library in SORMAS-Project, which moves from javax to the jakarta namespace in version 22.
Not updating to keycloak 22 yet, because we want to keep the version in sync with the library in SORMAS-Project, which moves from javax to the jakarta namespace in version 22.
Related PR in SORMAS: https://github.com/SORMAS-Foundation/SORMAS-Project/pull/12234