Update severity level filter

What does this MR do and why?

Update severity level filter

• show on default
• not be removed
• removes empty array from yaml if all are selected

Changelog: changed

EE: true

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
severity_level_before	severity_level_after

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Navigate to scan result policy editor rule mode
2. verify that severity_levels criteria UI shows with All severity levels selected in rule mode and the severity_levels: [] key is in the yaml
3. User selects specific severity
4. verify that severity_levels key appears in yaml with selected values in an array
5. User selects All severity levels
6. verify that severity_levels: [] key is in the yaml

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 #419682 (closed)