Bump urllib3 from 1.26.5 to 1.26.18 in /server/requirements

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.5 to 1.26.18.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](urllib3/urllib3@1.26.5...1.26.18)

---
updated-dependencies:
- dependency-name: urllib3
  dependency-type: direct:production
...

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

server/requirements/base.txt

@@ -37,7 +37,7 @@ sendgrid==6.4.7
 simplejson==3.17.5
 six==1.15.0
 sqlparse==0.3.1
-urllib3==1.26.5
+urllib3==1.26.18
 us==2.0.2
 uuid==1.30
 webencodings==0.5.1