replace debian sample systemd service file by readme

The sample service file is as of 2023-10-23 no longer provided by
the rsyslog project because service settings strongly depend on
distribution policies.

For Debian, the distro current version can be found at

https://salsa.debian.org/debian/rsyslog/-/blob/debian/master/debian/rsyslog.service

Note: We provided a sample service files in the early days of systemd,
when almost nobody was using systemd. That made sense because it eased
transition for those trying out systemd. Now that systemd is mainstream,
there is no real need any longer for a sample.

platform/debian/rsyslog.service.README

@@ -0,0 +1,12 @@
+The sample service file is as of 2023-10-23 no longer provided by
+the rsyslog project because service settings strongly depend on
+distribution policies.
+
+For Debian, the distro current version can be found at
+
+https://salsa.debian.org/debian/rsyslog/-/blob/debian/master/debian/rsyslog.service
+
+Note: We provided a sample service files in the early days of systemd,
+when almost nobody was using systemd. That made sense because it eased
+transition for those trying out systemd. Now that systemd is mainstream,
+there is no real need any longer for a sample.