Revert "base: Don't handle the wildcard case for member events anymore."

Using the exhaustive feature in ruma enables the appservice/federation
apis, adding some 10 more crates to our dependencies. Disable that
feature for now.

This reverts commit 41529a6bff.

matrix_sdk_base/src/models/room.rs

@@ -1091,6 +1091,7 @@ impl Describe for MembershipState {
             Self::Join => "is a member of",
             Self::Knock => "is requesting access to",
             Self::Leave => "has left",
+            _ => "unhandled case of MembershipState",
         }
         .to_string()
     }
@@ -1123,6 +1124,7 @@ impl Describe for MembershipChange {
             Self::None => "did nothing in",
             Self::NotImplemented => "NOT IMPLEMENTED",
             Self::Error => "ERROR",
+            _ => "unhandled case of MembershipChange",
         }
         .to_string()
     }