Fix issue self-hosted action-runners docs (#28487)

[chickenandpork]

Why:

Promotes minimum OS requirements to ensure visibility and replace value that can be misleading

Closes: #28487

What's being changed (if available, include any code snippets, screenshots, or gifs):

Action-Runners 2.309.0 continued the shift to newer nodes versions; in this case, node20. Documentation still says that the action-runner supports macOS-11.13 or later, but node20 only supports macOS-11.0 and later, introducing inaccuracy in GH docs.

See also https://github.com/nodejs/node/blob/main/BUILDING.md#platform-list, notably nodejs/node@4e39830 introduced in nodejs/node#49164

This one-liner elevates that new constraint.

Check off the following:

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline.

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

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[github-actions]

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
actions/hosting-your-own-runners/managing-self-hosted-runners/about-self-hosted-runners.md	fpt ghec ghes@ 3.10 3.9 3.8 3.7 3.6 ghae	fpt ghec ghes@ 3.10 3.9 3.8 3.7 3.6 ghae

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server
ghae: GitHub AE

[cmwilson21]

@chickenandpork Thanks for opening an issue and submitting a PR! I edited your post slightly to link it to your issue 👍

I'll get this triaged for review! ⚡

[jc-clark]

Thanks for updating this @chickenandpork!

I'll help get this merged 🚀

[github-actions]

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