doc: trim deprecation level definition text

Provide more concise explanations of the deprecation levels in
COLLABORATOR_GUIDE.md.

PR-URL: https://github.com/nodejs/node/pull/21241


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
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Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
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Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
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