Use a documentation-only IP to avoid confusion and follow best practice.
This prevents irritation when users or scanners see a private address in
samples and wonder if rsyslog is trying to contact it.
Impact: Visible change in sample payloads, tests, and docs (host field).
Technical: Replace the hardcoded "172.20.245.8" in imdiag's generated
messages with a constant default hostname "192.0.2.8" from RFC 5737
(TEST-NET-1). Update tests (e.g., omrabbitmq, perctile, mmexternal,
clickhouse) and template examples to match. No API/ABI change; no effect
on action queues, OMODTX, retry/duplicate semantics, or HUP behavior.
This is purely about sample message contents emitted by the diag tool.
Before/After: samples used 172.20.245.8 -> now 192.0.2.8.
See also: https://gitee.com/src-openeuler/rsyslog/issues/I7V4I8
Fixes: https://github.com/rsyslog/rsyslog/issues/6190
