The telemetry toggle in coder/coder is lacking tests and backwards compatibility

Discovered an inconsistency in our docs: https://github.com/coder/coder/pull/15467

While at it, I noticed we never test that this environment variable actually works, and we never did introduce back-compat when it was renamed from `CODER_TELEMETRY` to `CODER_TELEMETRY_ENABLE`. Both these issues will be fixed.