This PR will be used for the first 8.x Helm chart release, that will also be the last official Helm chart release (see https://github.com/elastic/helm-charts/issues/1731).

This release will be directly done from the `main` branch and no `8.x` or `8.5` branchs will be created as we don't expect to update the charts for new versions in the future.

- [x] bump version from 8.4.1 to 8.5.1
- [x] remove disclamer on chart README pages
- [x] add a new 8.x column to the list of supported versions in the global README page
- [x] add the official message for the last release
- [x] update the changelog and breaking changes files