Bump boto3 from 1.21.0 to 1.21.4 in /requirement

Bumps [boto3](https://github.com/boto/boto3) from 1.21.0 to 1.21.4.
- [Release notes](https://github.com/boto/boto3/releases)
- [Changelog](https://github.com/boto/boto3/blob/develop/CHANGELOG.rst)
- [Commits](boto/boto3@1.21.0...1.21.4)

---
updated-dependencies:
- dependency-name: boto3
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

requirement/backend_requirement.txt

@@ -3,7 +3,7 @@ jinja2==3.0.3
 triopg==0.6.0
 trio-asyncio==0.12.0
 # S3
-boto3==1.21.0
+boto3==1.21.4
 botocore==1.24.4
 # Swift
 python-swiftclient==3.13.0