[keycloak-dev] Migration guide moved!

[Stian Thorgersen]

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.