Dehydrated devices: API change POST->GET

[uhoreg]

use GET instead of POST for /events

will require

• updating the MSC
• updating the Synapse implementation
• updating Rust SDK (we think not because method doesn't touch rust-sdk)
• updating JS-SDK

Re-check breaking backwards compatibility with @mxandreas.

[mxandreas]

Since it is currently not known when this change will be made, breaking the backwards compatibility needs to be checked. It can happen that some users uptake this feature in a production before, and we would then avoid disruptions unless it becomes very expensive.