fix: docs/samples/kafka/transformer.Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-DEBIAN12-EXPAT-7855502
- https://snyk.io/vuln/SNYK-DEBIAN12-EXPAT-7855503
- https://snyk.io/vuln/SNYK-DEBIAN12-EXPAT-7855507
- https://snyk.io/vuln/SNYK-DEBIAN12-ZLIB-6008963
- https://snyk.io/vuln/SNYK-DEBIAN12-PAM-6178914

docs/samples/kafka/transformer.Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.9-slim
+FROM python:3.13.0rc2-slim
 
 RUN apt-get update && apt-get install -y libglib2.0-0
 RUN pip install --no-cache-dir --upgrade pip && pip install --no-cache-dir kserve