[CI] benchmark readme (#16783)

- add instruction to run benchmark in v8 with `xpack.monitoring.allow_legacy_collection`
- remove scripted field 5m_num from dashboards

.buildkite/scripts/benchmark/README.md

@@ -18,4 +18,5 @@
 ## Notes
 - Benchmarks should only be compared using the same hardware setup.
 - Please do not send the test metrics to the benchmark cluster. You can set `VAULT_PATH` to send data and metrics to your own server.
-- Run `all.sh` as calibration which gives you a baseline of performance in different versions.
+- Run `all.sh` as calibration which gives you a baseline of performance in different versions.
+- [#16586](https://github.com/elastic/logstash/pull/16586) allows legacy monitoring using the configuration `xpack.monitoring.allow_legacy_collection: true`, which is not recognized in version 8. To run benchmarks in version 8, use the script of the corresponding branch (e.g. `8.16`) instead of `main` in buildkite.

.buildkite/scripts/benchmark/save-objects/CHANGELOG.md

@@ -1,3 +1,6 @@
+## 20241210
+Remove scripted field `5m_num` from dashboards
+
 ## 20240912
 Updated runtime field `release` to return `true` when `version` contains "SNAPSHOT"