chore: Source Postgrey `whitelist_clients` config from Github (#3879)

Use a more updated list for Postgrey `whitelist_clients`

CHANGELOG.md

@@ -65,6 +65,8 @@ The most noteworthy change of this release is the update of the container's base
 - **Environment Variables:**
   - `ONE_DIR` has been removed (legacy ENV) ([#3840](https://github.com/docker-mailserver/docker-mailserver/pull/3840))
     - It's only functionality remaining was to opt-out of run-time state consolidation with `ONE_DIR=0` (_when a volume was already mounted to `/var/mail-state`_).
+- **Internal:**
+  - Changed the Postgrey whitelist retrieved during build to  [source directly from Github](https://github.com/schweikert/postgrey/blob/master/postgrey_whitelist_clients) as the list is updated more frequently than the [author publishes at their website](https://postgrey.schweikert.ch) ([#3879](https://github.com/docker-mailserver/docker-mailserver/pull/3879))
 - **Tests:**
   - Refactored helper methods for sending e-mails with specific `Message-ID` headers and the helpers for retrieving + filtering logs, which together help isolate logs relevant to specific mail when multiple mails have been processed within a single test. ([#3786](https://github.com/docker-mailserver/docker-mailserver/pull/3786))
 - **Rspamd**:

Dockerfile

@@ -144,7 +144,7 @@ COPY target/postgrey/postgrey /etc/default/postgrey
 RUN <<EOF
   mkdir /var/run/postgrey
   chown postgrey:postgrey /var/run/postgrey
-  curl -Lsfo /etc/postgrey/whitelist_clients https://postgrey.schweikert.ch/pub/postgrey_whitelist_clients
+  curl -Lsfo /etc/postgrey/whitelist_clients https://raw.githubusercontent.com/schweikert/postgrey/master/postgrey_whitelist_clients
 EOF
 
 COPY target/amavis/conf.d/* /etc/amavis/conf.d/