mix.exs moves to 2.1.0: a minor bump, since ADR-0067's driver-agnostic
telemetry contract (st-737e, PR #224) adds the public
Statifier.Telemetry module and breaks nothing -
Statifier.Session.Telemetry keeps its functions and arities as a
facade.

CHANGELOG.md gains the [2.1.0] section, assembled from the one
changelog.d/ fragment (st-737e), which is deleted in this commit per
changelog.d/README.md. The README install pin (~> 2.0) already covers
2.1.0 and is unchanged.

Tagging v2.1.0 happens on main after the merge; mix hex.publish stays
the operator's action.

Refs: st-ipna