Hardcode postgres version in docker-compose files

This is mostly necessary for the `./cms-dev.sh` use case, since the default postgres image was updated and now we're getting an error when starting the container with the old data.

Also, a minor fix to SUPPORT.md (last reference of gitter -> telegram)

Author: wil93

.github/SUPPORT.md

@@ -20,5 +20,5 @@ If you encountered a bug, please file an
 the instructions in the issue template.
 
 **Please don't file issues to ask for help**, we are happy to help
-on the mailing list or on gitter, and it is more likely somebody will
+on the mailing list or on Telegram, and it is more likely somebody will
 answer your query sooner.

docker-compose.dev.yml

@@ -1,6 +1,6 @@
 services:
   devdb:
-    image: postgres
+    image: postgres:15
     environment:
       POSTGRES_HOST_AUTH_METHOD: trust
     volumes:

docker-compose.test.yml

@@ -1,6 +1,6 @@
 services:
   testdb:
-    image: postgres
+    image: postgres:15
     environment:
       POSTGRES_HOST_AUTH_METHOD: trust