Bump protobuf from 3.6.0 to 3.15.0 in /examples/yoti_example_flask

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.6.0 to 3.15.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.6.0...v3.15.0)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: indirect
...

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

Author: dependabot[bot]

examples/yoti_example_flask/requirements.txt

@@ -40,7 +40,7 @@ jinja2==3.0.3
     #   flask
 markupsafe==2.0.1
     # via jinja2
-protobuf==3.6.0
+protobuf==3.15.0
     # via yoti
 pycparser==2.18
     # via cffi
@@ -74,6 +74,3 @@ wrapt==1.12.1
     # via deprecated
 yoti==2.14.0
     # via -r requirements.in
-
-# The following packages are considered to be unsafe in a requirements file:
-# setuptools