Remove Ubuntu 20.04 from supported GitHub Actions runners list

[Sebastian-Webster]

Why:

Ubuntu 20.04 is now fully unsupported (see this issue) and references to it has been removed from the actions/runner-images repository in this PR

Closes: #37759

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

This pull request removes Ubuntu 20.04 from the supported GitHub-hosted runners list in the docs as it is now fully unsupported.

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.

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Table of review links

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

Source	Review	Production	What Changed
actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners.md	fpt ghec ghes@ 3.16 3.15 3.14 3.13 3.12	fpt ghec ghes@ 3.16 3.15 3.14 3.13 3.12	from reusable

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

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[Sharra-writes]

Thanks for opening a PR! I'll get this triaged for review.

[Sharra-writes]

Okay, this looks good! I'll get it added to the merge queue once checks are passing. ✨

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