Updates "Vulnerability resolution" feature name to match marketing docs

What does this MR do and why?

It aligns the name of the Vulnerability Management AI features, Vulnerability Resolution and Vulnerability Explanation, to match the sentence case used on https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html#gitlab-duo.

While I think these features should be regarded as proper nouns, such as GitLab Duo, and thus should be title case (capitalizing both words), @sselhorn (a Staff Technical Writer) informed me of the following in Slack (internal):

We debated these things and at the end of the day had to follow the capitalization that Marketing is using. The capitalization is inconsistent, but the word list you linked to is the source of truth. We've had some further debates (including here) and others have mentioned the inconsistency. (As a writer, it makes me  ). Long term we won't need official feature names and can just describe what the product does. But because we're trying to help with adoption, we've been trying to keep consistent with these Marketing terms.

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Before	After
Vulnerability Resolution in some places, and Vulnerability resolution in other places.	Vulnerability resolution

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