Bump debian in /images/prow-tests

Bumps debian from bullseye-20250811-slim to bullseye-20250908-slim.

---
updated-dependencies:
- dependency-name: debian
  dependency-version: bullseye-20250908-slim
  dependency-type: direct:production
...

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

Author: dependabot[bot]

images/prow-tests/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM debian:bullseye-20250811-slim AS base
+FROM debian:bullseye-20250908-slim AS base
 
 # Pinned versions of stuff we pull in
 ARG TARGETARCH