⬆️ Bump watchdog from 2.1.7 to 6.0.0 in /tools/serve_header

Bumps [watchdog](https://github.com/gorakhargosh/watchdog) from 2.1.7 to 6.0.0.
- [Release notes](https://github.com/gorakhargosh/watchdog/releases)
- [Changelog](https://github.com/gorakhargosh/watchdog/blob/master/changelog.rst)
- [Commits](gorakhargosh/watchdog@v2.1.7...v6.0.0)

---
updated-dependencies:
- dependency-name: watchdog
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

tools/serve_header/requirements.txt

@@ -1,2 +1,2 @@
 PyYAML==6.0
-watchdog==2.1.7
+watchdog==6.0.0