Update Debian Stretch OS to Buster

[maya-ross]

Update Debian Stretch OS to Buster as Buster is the OS from GHES 3.2.

Why:

Updates the OS version to the correct version, Debian Buster, from GHES 3.2.

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

Check off the following:

• I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• For content changes, I have completed the self-review checklist.

[github-actions]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

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
admin/overview/system-overview.md	ghes@ 3.7 3.6 3.5 3.4 3.3	ghes@ 3.7 3.6 3.5 3.4 3.3

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

[github-actions]

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