Bump ddtrace from 1.1.4 to 1.6.4 in /backend

Bumps [ddtrace](https://github.com/DataDog/dd-trace-py) from 1.1.4 to 1.6.4.
- [Release notes](https://github.com/DataDog/dd-trace-py/releases)
- [Changelog](https://github.com/DataDog/dd-trace-py/blob/1.x/CHANGELOG.md)
- [Commits](https://github.com/DataDog/dd-trace-py/compare/v1.1.4...v1.6.4)

---
updated-dependencies:
- dependency-name: ddtrace
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

backend/requirements.txt

@@ -9,4 +9,4 @@ pytest==7.1.2
 requests==2.28.0
 discord-webhook==0.16.3
 datadog==0.44.0
-ddtrace==1.1.4
+ddtrace==1.6.4