Bump urllib3 from 1.26.15 to 1.26.18 in /tools/load-generator

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.15 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@1.26.15...1.26.18)

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

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

Author: dependabot[bot]

tools/load-generator/requirements.txt

@@ -3,4 +3,4 @@ certifi==2023.5.7
 idna==3.4
 prometheus-client==0.17.0
 requests==2.28.2
-urllib3==1.26.15
+urllib3==1.26.18