Bot fails to respond after matrix-js-sdk update due to private property access

[xurizaemon]

Hi there

There's an access to a private property in src/matrix.js#L165 which is no longer compatible with the matrix-js-sdk after v12.

(The relevant change happens in matrix-org/matrix-js-sdk#1718 where the property is renamed; in TS it is marked private properly.)

IMO we should:

• Fix the private property access (which was risky before this change anyway) - Drop use of private SDK property room.currentState._userIdsToDisplayNames #24
• Constrain the required adapter to known-good version(s) - Matrix JS SDK v12 #25 or Require matrix-js-sdk@^9.11.0 (per current package-lock.json) #26