Benchmark: make summary.sh reusable, move notes into the workflow

- Parametrize summary.sh: the server name, version and image are now arguments
  per server (`<name> <statistics.json> <version> <image>` x2), replacing the
  hardcoded "OpenLDAP"/"OpenDJ". The report title, tables and chart
  legends/labels are all driven by the passed names, so the script can compare
  any two LDAP servers.
- Move the benchmark-specific Notes out of the script into the workflow's
  "Build job summary" step (appended to $GITHUB_STEP_SUMMARY after the generic
  report).