Fix M_USER_ACCOUNT_SUSPENDED error code for spec compliance (#12922)

`M_` is a reserved namespace.

changelog.d/12845.feature

-Support the new error code "M_ORG_MATRIX_MSC3823_USER_ACCOUNT_SUSPENDED" from [MSC3823](https://github.com/matrix-org/matrix-spec-proposals/pull/3823).
+Support the new error code "ORG.MATRIX.MSC3823.USER_ACCOUNT_SUSPENDED" from [MSC3823](https://github.com/matrix-org/matrix-spec-proposals/pull/3823).
\ No newline at end of file

changelog.d/12922.feature

+Support the new error code "ORG.MATRIX.MSC3823.USER_ACCOUNT_SUSPENDED" from [MSC3823](https://github.com/matrix-org/matrix-spec-proposals/pull/3823).
\ No newline at end of file

synapse/api/errors.py

@@ -84,7 +84,7 @@ class Codes(str, Enum):
     # By opposition to `USER_DEACTIVATED`, this is a reversible measure
     # that can possibly be appealed and reverted.
     # Part of MSC3823.
-    USER_ACCOUNT_SUSPENDED = "M_ORG_MATRIX_MSC3823_USER_ACCOUNT_SUSPENDED"
+    USER_ACCOUNT_SUSPENDED = "ORG.MATRIX.MSC3823.USER_ACCOUNT_SUSPENDED"
 
     BAD_ALIAS = "M_BAD_ALIAS"
     # For restricted join rules.