[Security Solution][Serverless][8.16] Suppression isn't labeled as optional for Threshold rules

[nastasha-solomon]

Describe the bug:

The Suppress alerts field isn't labeled as optional in the rule creation form for Threshold rules.

Kibana/Elasticsearch Stack version: 8.16

Server OS version: N/A

Browser and Browser OS versions: N/A

Elastic Endpoint version: N/A

Original install method (e.g. download page, yum, from source, etc.): N/A

Functional Area (e.g. Endpoint management, timelines, resolver, etc.): Rules

Steps to reproduce:

1. Go to Rules → Detection rules (SIEM) → Create new rule.
2. Select Threshold.
3. Scroll down to the Suppress alerts field.

Current behavior:
The Suppress alerts field isn't labeled as optional.

Expected behavior:
The Suppress alerts field should be labeled as optional.

Screenshots (if relevant): See above

Errors in browser console (if relevant): N/A

Provide logs and/or server output (if relevant):

Any additional context (logs, chat logs, magical formulas, etc.):

[elasticmachine]

Pinging @elastic/security-detection-engine (Team:Detection Engine)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)