Fix display issues with Testing releases on What's New and Releases index

[mdlinville]

• Prevent Feature Highlights and related links from showing up in What's New and Releases index prior to GA
• Improve logic in Releases index to determine LTS and skippable releases status
• Add back the details about Testing releases not being supported to What's New and Releases index, borrowing language from Release Support Policy
• Fix CC variables to use the new plan names, fix some outdated info about Regular and Innovation releases on CC plans
• Fix another tiny typo
• Fix outdated future-tense detail in Releases index about v24.2, which is now released

Testing

• Local build, confirmed the details look right for v24.3, v24.2, v24.1, v23.2, v23.1, v22.2 (that's all the categories)
• Local linkcheck

Previews

releases/index.md#v24.3 src/current/releases/v24.3.md

Should show no change to output:

releases/index.md#v24.2 releases/index.md#v24.1 releases/index.md#v23.2 releases/index.md#v23.1 releases/index.md#v23.1 src/current/releases/v24.2.md src/current/releases/v24.1.md src/current/releases/v23.2.md src/current/releases/v23.1.md src/current/releases/v22.2.md

[github-actions[bot]]

Files changed:

• src/current/_includes/releases/whats-new-intro.md:
  • releases/releases.md
• src/current/releases/index.md
• src/current/releases/v24.3.md

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