Element X removes custom powerlevels since the update that added room version 12 support

### Steps to reproduce

NOTE: issue reported for matrix:u/beo:tchncs.de in matrix:roomid/HKMjUNHEZTjndeDqhI%3Aelement.io/%24MfNqVD2R5cAiVIPDXAL7Gc3a_VTNTH2h3eiSolCh7L0?via=matrix.org&via=element.io&via=one.ems.host
(they report this also may just always have been the case on EX)

1. View member list
2. Note that it only shows hardcoded levels
3. Attempt to update powerlevels
4. Notice that custom powerlevels are rounded down to the nearest hardcoded level

### Outcome

#### What did you expect?
Custom PLs are preserved
#### What happened instead?
Custom PLs are lost

### Your phone model

_No response_

### Operating system version

_No response_

### Application version and app store

_No response_

### Homeserver

_No response_

### Will you send logs?

No

### Are you willing to provide a PR?

No