Switch to the stable /hierarchy endpoint for spaces

[clokep]

The stable /hierarchy endpoint from MSC2946 has been available since Synapse v1.49.0 (released on 2021-12-14), see matrix-org/synapse#11329.

It would be great if clients could switch to using the stable API so that the unstable API can be removed from Synapse. Unfortunately there was a bug in Synapse which was fixed in Synapse v1.50.0 (see matrix-org/synapse#11602) to be released on 2022-01-11.

Two releases of Synapse usually gets most of the ecosystem to upgrade, so I suggest we wait until Synapse v1.52.0 is released (scheduled for 2022-02-07) for clients to switch.

• Element Android issue: Use the stable hierarchy endpoint from MSC2946 element-android#4462
• Element iOS issue: Use the stable hierarcy endpoint from MSC2946 element-ios#5144
• Element Web issue: Use the stable hierarchy endpoint from MSC2946 element-web#19738

[clokep]

Also see the Synapse tracking issue for disabling unstable support: matrix-org/synapse#10946

[clokep]

This is done!