Use updates stable relations from js-sdk (#8715)

* Use updates stable relations from js-sdk

* Relations is now accessible on the Room instead

* Reuse more existing code and ditch confusing `isThreadRelation`

* Fix last usage of removed `isThreadRelation`

* Update tests to match removal of isThreadRelation

* Tweak method naming to closer match spec

* Fix missing method name change

src/MatrixClientPeg.ts

@@ -285,7 +285,6 @@ class MatrixClientPegClass implements IMatrixClientPeg {
                 SHOW_QR_CODE_METHOD,
                 verificationMethods.RECIPROCATE_QR_CODE,
             ],
-            unstableClientRelationAggregation: true,
             identityServer: new IdentityAuthClient(),
             cryptoCallbacks: {},
         };