Clarify runner usage for GitHub Enterprise Server users

[mcummings128]

Added note about GitHub Enterprise Server users needing to use self-hosted runners instead of GitHub-hosted runners.

Why:

Closes: Issue #44020

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

Adds note regarding that GHES instances cannot use Github-hosted runners. This is mentioned in the How-tos: Manage runners: Github-hosted runners documentation, and should be mentioned here for conformity and another way to inform potential users of GHES that Github-hosted runners cannot be used

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
actions/concepts/runners/github-hosted-runners.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

🤖 This comment is automatically generated.

[Sharra-writes]

@mcummings128 This is fine. We have a reusable for it, though, so I just substituted that in. I'll get this approved.

[github-actions]

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[mcummings128]

Thank you!!