[docs] Update "Display beta integrations" guidance for Kibana 9.4+ (toggle removed)

## Summary

Several integration docs instruct users to enable **"Display beta integrations"** on the Kibana Integrations page as if that control always exists. The toggle was **removed in Kibana 9.4+** ([elastic/kibana#252115](https://github.com/elastic/kibana/pull/252115)), so current wording is wrong for 9.4/9.5 and confuses customers who cannot find the control.

We should **not** drop version context entirely: behaviour differs by Kibana version, and docs should say so clearly.

## Behaviour by version
| Kibana version | How to see / use beta (prerelease) integrations |
| --- | --- |
| **< 9.4** | The **"Display beta integrations"** toggle on Browse integrations is required (enable it beneath the category list). |
| **9.4+** | The toggle no longer exists. Many beta / technical-preview packages are searchable by default. |

Reference: [elastic/kibana#252115](https://github.com/elastic/kibana/pull/252115) (`v9.4.0`).

## Affected documentation (known)
1. [Beelzebub](https://www.elastic.co/docs/reference/integrations/beelzebub) — technical preview admonition *and* setup step that always says to enable the toggle.
Source: `packages/beelzebub/docs/README.md` / `_dev/build/docs/README.md`
2. [Elastic Package Registry](https://www.elastic.co/docs/reference/integrations/elastic_package_registry) — technical preview admonition with the same always-on toggle instruction.

The technical-preview callout looks shared/templated across packages. A repo-wide / docs-pipeline search for `Display beta integrations` is recommended so we fix the generator/template once where possible.

Search the repository and docs pipeline for “Display beta integrations,” starting with packages/beelzebub/docs/README.md and _dev/build/docs/README.md, then inspect the shared technical-preview template if the wording is generated. Preserve the version distinction: pre-9.4 requires the toggle, while 9.4+ does not. Verify the affected Beelzebub and Elastic Package Registry documentation no longer gives an unconditional instruction.

Written by the indexing model from the issue text.