[srvls] Event Sources should be under Serverless group together with Channel

These bugs change the "Serverless" add group to "Eventing" and moves the "Event Sources" add option from "Developer Catalog" to the "Eventing" add to the group. This was also backported to 4.8.

• https://bugzilla.redhat.com/show_bug.cgi?id=1992820
• https://bugzilla.redhat.com/show_bug.cgi?id=1999931