Ignore message in Vulnerabilities::Finding

What does this MR do and why?

Ignore messages in Vulnerabilities::Finding and remove its use everywhere except on public interfaces.

This field was deprecated as part of Cut and release security report schema version ... (#363134 - closed). Its use in public interfaces wasn't deprecated before 16.0, so we won't remove those until 17.0. The fields will return nil until then though.

Related to #411575 (closed)

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Related to #411575 (closed)