Rephrase error messages

What does this MR do and why?

This MR rephrases error messages in the vulnerability form.

Screenshots or screen recordings

before	after

How to set up and validate locally

1. You'll need an EE license.
2. Navigate to :namespace/:project/-/security/vulnerabilities/new using the Address bar
3. Hit submit button without filling any input

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #351866 (closed)