[docs] Add CODEOWNERS suggestion to CONTRIBUTING.md

[steveburnett]

Description

• Add to Committers in CONTRIBUTING.md a suggestion encouraging readers to volunteer as codeowners for areas of Presto lacking active codeownership.
• Delete an empty Requirements heading (the Presto Requirements have been moved to README.md) in CONTRIBUTING.md that was overlooked in Add table for users to quickly navigate documentation #22930.

Motivation and Context

The first addresses a suggestion that I made in #22641. The second fixes a text error.

Impact

Documentation.

Test Plan

Pushed branch to my fork of the repo and View File to see the file as it will be displayed in GitHub. Also, CI.

• Screenshot of new Committers topic, showing the added text in the two sentences beginning with "See CODEOWNERS for modules, connectors, [...]".

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• Screenshot of current CONTRIBUTING.md, showing the empty Requirements heading that this PR deletes.

Contributor checklist

• Please make sure your submission complies with our development, formatting, commit message, and attribution guidelines.
• PR description addresses the issue accurately and concisely. If the change is non-trivial, a GitHub Issue is referenced.
• Documented new properties (with its default value), SQL syntax, functions, or other functionality.
• If release notes are required, they follow the release notes guidelines.
• Adequate tests were added if applicable.
• CI passed.

Release Notes

== NO RELEASE NOTE ==

[tdcmeehan]

I opened #22942 which includes this, will close in favor of that PR @steveburnett!