guests.md: Focal Fossa is not the last ubuntu having guest tools anymore

[stormi]

Supported versions of Ubuntu have guest tools in their repositories.

Before submitting the pull request, you must agree with the following statements by checking both boxes with a 'x'.

• "I accept that my contribution is placed under the CC BY-SA 2.0 license [1]."
• "My contribution complies with the Developer Certificate of Origin [2]."

[1] https://creativecommons.org/licenses/by-sa/2.0/
[2] https://xcp-ng.org/docs/contributing.html#developer-certificate-of-origin-dco

[ydirson]

We may still want to mention that a few releases did not ship them?

[stormi]

We may still want to mention that a few releases did not ship them?

Done