[keycloak-dev] Migration guide moved!

The upgrade guide from RH-SSO has been moved to keycloak-documentation and now serves as the upgrade guide for both Keycloak and RH-SSO. That means add changes for Keycloak to upgrading/topics/keycloak/changes.adoc and not to the old MigrationFromOlderVersions.adoc file. I've left the old MigrationFromOlderVersions.adoc for now in case there's something that's missing in the new guide, but it's not included in the topics list.