Clarify use of labels and groups together

[Nuru]

Why:

• Saying "and/or" when in fact the requirement is "and" is misleading.
• Saying "ARC does not support multiple labels" in an article that explains how to use multiple labels with ARC is extra confusing. See Documentation confuses Runner Scale Set Controller with Actions Runner Controller #35714 for more details.

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

As correctly explained under Using labels and groups to route jobs, when you combine groups and labels, the runner must meet both requirements to be eligible to run the job. Therefore, a statement like "GitHub looks for a runner that matches the job's runs-on labels and/or groups" is confusing and wrong. The runner must match the job's labels and groups and will not be selected if it only matches the job's labels or groups but not both. So all the "and/or" instances have been changed to "and".

Reference to ARC at the top of the article has been changed to a reference to Runner Scale Sets for accuracy. The Note has been upgraded to Important because the entire article does not apply if you are using Runner Scale Sets.

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 a 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.

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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

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Source	Preview	Production	What Changed
actions/hosting-your-own-runners/managing-self-hosted-runners/using-self-hosted-runners-in-a-workflow.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10

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

[nguyenalex836]

@Nuru Thanks so much for opening a PR! I'll get this triaged for review ✨