Target Redis 6 everywhere rather than a mix of 5 and 6 (#35373)

Redis 5 has been EOL for a few years, and didn't get updates for the
latest high-severity CVEs. We're already using 6 in most places
(fleetctl preview, recommended reference architectures, managed cloud
environments) so it's safe to set 6 as the new minimum.

Co-authored-by: Noah Talerman <47070608+noahtalerman@users.noreply.github.com>

docker-compose.yml

@@ -112,7 +112,7 @@ services:
       - ServerOptions__TlsCertificatePrivateKey=/certs/fleet.key
 
   redis:
-    image: redis:5
+    image: redis:6
     ports:
       - "6379:6379"