Add validation for timezone in scan execution policies

What does this MR do and why?

This MR adds validation for timezone in scan execution policies.

Screenshots or screen recordings

How to set up and validate locally

1. Navigate to Security & Compliance -> Policies -> New policy -> Scan execution policy

2. Switch to .yaml mode

3. Paste a policy with timezone. Example:

   type: scan_execution_policy
   name: Test with TZ
   description: ''
   enabled: true
   rules:
     - type: schedule
       branches:
         - main
       cadence: 0 0 * * *
       timezone: invalid
   actions:
     - scan: dast
       tags: [ ]
       site_profile: Default
       scanner_profile: Default      

4. Error should be displayed after submitting such policy

5. Changing the timezone to a valid timezone, for example America/New_York, Europe/Amsterdam, or UTC should submit successfully

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 #398259 (closed)