chore(deps): update dependency certifi to v2026 (#8829)

Author: renovate-bot

.kokoro/docs/docker/requirements.txt

@@ -8,9 +8,9 @@ cachetools==6.2.0 \
     --hash=sha256:1c76a8960c0041fcc21097e357f882197c79da0dbff766e7317890a65d7d8ba6 \
     --hash=sha256:38b328c0889450f05f5e120f56ab68c8abaf424e1275522b138ffc93253f7e32
     # via google-auth
-certifi==2025.10.5 \
-    --hash=sha256:0f212c2744a9bb6de0c56639a6f68afe01ecd92d91f14ae897c4fe7bbeeef0de \
-    --hash=sha256:47c09d31ccf2acf0be3f701ea53595ee7e0b8fa08801c6624be771df09ae7b43
+certifi==2026.1.4 \
+    --hash=sha256:9943707519e4add1115f44c2bc244f782c0249876bf51b6599fee1ffbedd685c \
+    --hash=sha256:ac726dd470482006e014ad384921ed6438c457018f4b3d204aea4281258b2120
     # via requests
 charset-normalizer==2.1.1 \
     --hash=sha256:5a3d016c7c547f69d6f81fb0db9449ce888b418b5b9952cc5e6e66843e9dd845 \