Bump pip from 18.1 to 21.1 in /utils/google_app_engine

Bumps [pip](https://github.com/pypa/pip) from 18.1 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@18.1...21.1)

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

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

Author: dependabot[bot]

utils/google_app_engine/additional_requirements.txt

@@ -1,4 +1,4 @@
 # add these requirements in your app on top of the existing ones
-pip==18.1
+pip==21.1
 Flask==1.0.2
 gunicorn==19.9.0