#501 feature flags missing from documentation

[martyav]

Fixes #501

I also did a refresh on the page, for clarity and to conform more closely to the style guides.

• Note about deprecated flags possibly being listed in the UI if you've upgraded from a previous Rancher version.
• Addition of rke1-custom-node-cleanup
• Flags were re-arranged in alphabetical order to match the UI.
• Style updates, such as contractions, shorter sentences, etc.
• Which version the flag applies to is added to some flags, in parentheses.
• Updates to the table, such as:
  • Arranging items alphabetically, as they appear in the Rancher UI
  • Only listing features that were added in the previous major release or later (exception -- fleet, which underwent a major change in behavior between 2.5 and 2.6)
  • Removing the empty "Restart Required" column, with only had marks for 2 deprecated features

Changes applied to 2.7, 2.6, and 2.5 pages. I left the oldest page alone as the changes weren't as relevant.

[martyav]

Upon checking the rancher/rancher repo and release notes, I realized that proxy and dashboard are no longer available on fresh Rancher installs. It would seem they would only be visible if the user has upgraded from a Rancher version from before their removal from the UI.

[martyav]

Per rancher/rancher#37142 (comment) I was able to get the information I needed for rke1

[martyav]

@snasovich Could you do a tech review on feature-flags.md (or ping someone who would be a good fit)?