chore(deps): bump virtualenv in /.kokoro/docker/docs

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.26.0 to 20.26.6.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.26.0...20.26.6)

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

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

.kokoro/docker/docs/requirements.txt

@@ -32,7 +32,7 @@ platformdirs==4.2.1 \
     --hash=sha256:031cd18d4ec63ec53e82dceaac0417d218a6863f7745dfcc9efe7793b7039bdf \
     --hash=sha256:17d5a1161b3fd67b390023cb2d3b026bbd40abde6fdb052dfbd3a29c3ba22ee1
     # via virtualenv
-virtualenv==20.26.0 \
-    --hash=sha256:0846377ea76e818daaa3e00a4365c018bc3ac9760cbb3544de542885aad61fb3 \
-    --hash=sha256:ec25a9671a5102c8d2657f62792a27b48f016664c6873f6beed3800008577210
-    # via nox
+virtualenv==20.26.6 \
+    --hash=sha256:280aede09a2a5c317e409a00102e7077c6432c5a38f0ef938e643805a7ad2c48 \
+    --hash=sha256:7345cc5b25405607a624d8418154577459c3e0277f5466dd79c49d5e492995f2
+    # via nox