[2027] Remove deprecated `deactivation` flag on `set_displayname` Module API

Deprecated since v1.150.0 in https://github.com/element-hq/synapse/pull/19542

Nobody is expected to be using this flag, except by accident.

We should enforce that all future arguments on this method, if any, should be kwarg-only, so that this positional argument doesn't risk being confused in the future.

Given the low pain on our side, I've set a slow removal date of 2027.

Start by locating the set_displayname Module API and review the deprecation context in PR #19542. Confirm how the deactivation flag is currently accepted and verify that future arguments are keyword-only; done means the deprecated flag is removed without ambiguity for remaining callers.

Written by the indexing model from the issue text.