chore(deps): bump urllib3 from 2.5.0 to 2.6.0 in /.kokoro/docs/docker (#8783)

Bumps [urllib3](https://github.com/urllib3/urllib3) from 2.5.0 to 2.6.0.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](urllib3/urllib3@2.5.0...2.6.0)

---
updated-dependencies:
- dependency-name: urllib3
  dependency-version: 2.6.0
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.kokoro/docs/docker/requirements.txt

@@ -140,7 +140,7 @@ rsa==4.9.1 \
     --hash=sha256:68635866661c6836b8d39430f97a996acbd61bfa49406748ea243539fe239762 \
     --hash=sha256:e7bdbfdb5497da4c07dfd35530e1a902659db6ff241e39d9953cad06ebd0ae75
     # via google-auth
-urllib3==2.5.0 \
-    --hash=sha256:3fc47733c7e419d4bc3f6b3dc2b4f890bb743906a30d56ba4a5bfa4bbff92760 \
-    --hash=sha256:e6b01673c0fa6a13e374b50871808eb3bf7046c4b125b216f6bf1cc604cff0dc
+urllib3==2.6.0 \
+    --hash=sha256:c90f7a39f716c572c4e3e58509581ebd83f9b59cced005b7db7ad2d22b0db99f \
+    --hash=sha256:cb9bcef5a4b345d5da5d145dc3e30834f58e8018828cbc724d30b4cb7d4d49f1
     # via requests