Move only and except to deprecated keywords

What does this MR do?

As mentioned in #427707 (comment 1631127660), we should move the only and except keywords to the deprecated section, even though we're not sure when we'll be able to actually remove them.

This also updates the note for deprecated items to make it clear that they are deprecated, but still functional for backwards compatibility.

I also deleted the unused commented-out troubleshooting section at the end of the file as it's not needed.

Related issues

• Resolves #427707 (closed), #429041 (closed)
• Related to https://gitlab.com/gitlab-org/technical-writing/-/issues/936

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