Bump debian in /dockerfiles/test-forward-proxy

Bumps debian from 13.6-slim to 13.7-slim.

---
updated-dependencies:
- dependency-name: debian
  dependency-version: 13.7-slim
  dependency-type: direct:production
...

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

dockerfiles/test-forward-proxy/Dockerfile

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 # Use a runtime image based on Debian slim
-FROM debian:13.6-slim
+FROM debian:13.7-slim
 
 # Install Squid and drop in a very basic, open proxy configuration.
 RUN apt-get update && apt-get install -y squid