Upgrade Keycloak version to 26.x

[mesembria]

Keycloak has released a major upgrade, version 26.0.0. We should upgrade our infrastructure to use the latest version.

https://www.keycloak.org/docs/latest/upgrading/index.html#migrating-to-26-0-0

Noting some changes we need to make when upgrading:

In the Keycloak theme, change the paths of external resources
-styles=web_modules/@fontawesome/fontawesome-free/css/icons/all.css web_modules/@patternfly/react-core/dist/styles/base.css web_modules/@patternfly/react-core/dist/styles/app.css node_modules/patternfly/dist/css/patternfly.min.css node_modules/patternfly/dist/css/patternfly-additions.min.css css/login.css css/styles.css
+styles=web_modules/@fontawesome/fontawesome-free/css/icons/all.css web_modules/@patternfly/react-core/dist/styles/base.css web_modules/@patternfly/react-core/dist/styles/app.css vendor/patternfly-v3/css/patternfly.min.css vendor/patternfly-v3/css/patternfly-additions.min.css css/login.css css/styles.css
In the helm chart, remove proxy: edge and replace it with proxyHeaders https://www.keycloak.org/docs/latest/upgrading/index.html#deprecated-proxy-option
(TBD) We may need to change how we set up an admin user, see https://www.keycloak.org/server/bootstrap-admin-recovery and https://www.keycloak.org/docs/latest/upgrading/index.html#admin-bootstrapping-and-recovery