Decouple policy name update from yaml/rule modes

What does this MR do and why?

Decouple policy name update from yaml/rule modes and set rule mode as default.

Related issue: #354572 (closed) and #354216 (closed)

Screenshots or screen recordings

Policy name is set from rule mode so only empty validation is present

Rule mode is the default mode

How to set up and validate locally

1. Enable scan_result_policy feature flag (if you haven't pulled master for a while)
2. Create a security project as described in the docs
3. Create a new scan result policy with the project owner

MR acceptance checklist

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