tests/multi-server: pin redpanda to v24.3.15

`:latest` resolves to v26.1.6, which aborts during seastar reactor
init with "close() syscall failed: Invalid argument" on the
self-hosted CI runners - even with the container running privileged
(so seccomp/AppArmor/capability bounding are all off).  That's a
regression in the image itself, not a sandbox problem.

Pin to v24.3.15 which starts cleanly.  Bump when a newer tag is
verified to work.

Author: arr2036

src/tests/multi-server/environments/kafka.yml.j2

@@ -30,7 +30,12 @@ x-common-config: &id001
     TEST_SUBNET: {{ test_subnet | default('172.16.0.0/12') }}
 services:
   kafka:
-    image: docker.redpanda.com/redpandadata/redpanda:latest
+    # Pinned to a specific 24.3.x release instead of :latest - the
+    # current :latest (v26.1.6) hits a close() EINVAL during seastar
+    # reactor init on the self-hosted CI runners even when the
+    # container runs privileged.  Bump when a newer tag is verified
+    # to start cleanly on the runner.
+    image: docker.redpanda.com/redpandadata/redpanda:v24.3.15
     # Redpanda's seastar reactor aborts during init under the default
     # Docker sandbox on self-hosted CI runners (close() EINVAL on an
     # internal fd).  Run privileged so the broker starts reliably; it's