Remove JoinRule.Private from the codebase (#6129)

It shouldn't be in use, since it was never properly defined:

- The `Private` values coming from the SDK are considered to be `Invite` instead.
- Remove any existing `JoinRule.Private` usages in our code.

features/roomdetails/impl/src/test/kotlin/io/element/android/features/roomdetails/impl/RoomDetailsPresenterTest.kt

@@ -582,7 +582,7 @@ class RoomDetailsPresenterTest {
             assertThat(awaitItem().canShowKnockRequests).isFalse()
             featureFlagService.setFeatureEnabled(FeatureFlags.Knock, true)
             assertThat(awaitItem().canShowKnockRequests).isTrue()
-            room.givenRoomInfo(aRoomInfo(joinRule = JoinRule.Private))
+            room.givenRoomInfo(aRoomInfo(joinRule = JoinRule.Invite))
             assertThat(awaitItem().canShowKnockRequests).isFalse()
             cancelAndIgnoreRemainingEvents()
         }