greying out versions when they don't apply

## Summary

Currently, when a feature isn't in a particular version, I create a tab section anyway, with a note. Users seem to like that, but have requested that we grey out those versions (rather than leaving them out), so they know right away that this feature doesn't apply to the greyed out version.

## Process

[List the steps to replicate the issue.]

## Current and expected result

[Describe what happened and what you expected.]

## Screenshot

[If relevant, please include a screenshot.]

The issue does not name any files, tests, or entry points. Start by locating the JavaScript entry point that parses Discourse RAD elements and controls version tabs, then reproduce a feature that is absent from one version; done means the unavailable version remains visible but is greyed out.

Written by the indexing model from the issue text.