Enable sharing historical keys on invite

After discussion with Product, we're ready to enable this key sharing work from
#5763 (based on
matrix-org/matrix-spec-proposals#3061).

src/settings/Settings.ts

@@ -224,7 +224,7 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         isFeature: true,
         displayName: _td("Share decryption keys for room history when inviting users"),
         supportedLevels: LEVELS_FEATURE,
-        default: false,
+        default: true,
     },
     "advancedRoomListLogging": {
         // TODO: Remove flag before launch: https://github.com/vector-im/element-web/issues/14231