Rename benchmark workflow and fix OpenLDAP version-capture SIGPIPE

Renames the workflow and its assets, and fixes the job failing before the
benchmark ran.

- Rename .github/workflows/performance.yml -> benchmark.yml (workflow name
  "LDAP benchmark: OpenDJ vs OpenLDAP") and .github/performance/ ->
  .github/benchmark/; update all internal paths and the concurrency group.
- Fix the "Capture OpenLDAP version" step failing with exit 141: `slapd -VV`
  piped to `head -1` got SIGPIPE when head closed the pipe early, and under
  `pipefail` that failed the whole step before any JMeter run. Capture the
  output fully and take the first line in bash, with a fallback to the image
  reference.
- Harden the OpenDJ version step the same way: guard ldapsearch with `|| true`
  so a transient bind failure cannot trip pipefail.