Four feature flags missing from documentation

[dnoland1]

Summary:
The current feature flag documentation at https://ranchermanager.docs.rancher.com/getting-started/installation-and-upgrade/installation-references/feature-flags lists 9 feature flags. However, in the Rancher v2.7.1 UI, there are 12:

Missing features flags (dashboard, proxy, rke1-custom-node-cleanup) should be documented.

[martyav]

I started a PR to address, but upon further research, both proxy and dashboard are discontinued flags that are documented in the 2.5 docs but not anywhere later (https://ranchermanager.docs.rancher.com/v2.5/reference-guides/installation-references/feature-flags#docusaurus_skipToContent_fallback). They should only be visible if you've upgraded from a previous Rancher version where they were once available. rke1-custom-node-cleanup, on the other hand, was introduced in 2.6 and isn't discontinued. I'm working on adding it to the feature flag page, as well as a note about deprecated flags appearing from previous Rancher versions.