chore(deps): bump coder/coder from v2.20.2 to v2.21.0 in /heroku (#216)

Bumps [coder/coder](https://github.com/coder/coder) from v2.20.2 to v2.21.0.
- [Release notes](https://github.com/coder/coder/releases)
- [Changelog](https://github.com/coder/coder/blob/main/release.key)
- [Commits](coder/coder@v2.20.2...v2.21.0)

---
updated-dependencies:
- dependency-name: coder/coder
  dependency-version: v2.21.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

heroku/Dockerfile

@@ -1,3 +1,3 @@
-FROM ghcr.io/coder/coder:v2.20.2
+FROM ghcr.io/coder/coder:v2.21.0
 ENV CODER_TELEMETRY_INSTALL_SOURCE=heroku
 ENTRYPOINT []