doc: update deprecations info in COLLABORATOR_GUIDE

The deprecations section in the COLLABORATOR_GUIDE is a bit long-winded and repetitive. It also includes some aspirational items that appear worded to create "an impression that a specific or meaningful statement has been made, when instead only a vague or ambiguous claim has actually been communicated." (Putting it in quotes because I'm copying it from Wikipedia.)

This commit edits the text to make it more concise and meaningful.

Checklist

• documentation is changed or added
• commit message follows commit guidelines