Benchmark: lighter default load, log-scale latency, per-server log artifacts

- Lower the default load to 128 threads / 300s (was 256 / 600), in the workflow
  inputs and env fallbacks and in the JMX __P defaults.
- Latency chart: switch the Y axis to logarithmic so the small per-operation
  values stay readable next to the large ones (e.g. OpenLDAP BIND).
- Capture server logs and upload them as two per-server artifacts. Each Stop step
  (if: always(), so logs are kept on failure too) now saves the container's
  `docker logs` to <server>/server.log and copies the in-container log directory
  (OpenDJ /opt/opendj/data/logs -> opendj/internal; OpenLDAP /var/log ->
  openldap/var-log). Uploaded as artifacts logs-opendj and logs-openldap.
- Set retention-days: 90 on all artifacts (jmeter-reports, logs-openldap,
  logs-opendj).