Bump tornado from 5.1.1 to 6.4.1 in /dashboard

Bumps [tornado](https://github.com/tornadoweb/tornado) from 5.1.1 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v5.1.1...v6.4.1)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

dashboard/requirements.txt

@@ -1,6 +1,6 @@
 absl-py==0.9.0
 bokeh==2.3.2
-tornado==5.1.1
+tornado==6.4.1
 pandas==1.1.5
 pandas-gbq==0.13.1
 google-api-python-client==1.8.0