[Security Solution] Enable SIEM migrations in serverless (#214708)

## Summary

Enables the SIEM Migrations feature in serverless. The experimental flag
will be cleaned later.


> [!NOTE]
To release this feature as planned (Friday 28th March 2025), this PR
needs to merge between Monday **24** and Thursday **27**


### Screenshot


![siem_migrations](https://github.com/user-attachments/assets/e20e2dc9-77cb-4460-8da7-c5a83909d378)

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

config/serverless.security.yml

@@ -180,7 +180,6 @@ xpack.index_management.enableProjectLevelRetentionChecks: true
 # These features are disabled in Serverless until fully tested
 xpack.securitySolution.enableExperimental:
   - entityStoreDisabled
-  - siemMigrationsDisabled
 
 # Increase task manager capacity because security projects have more resources (Memory and CPU)
 xpack.task_manager.capacity: 20

x-pack/test/security_solution_api_integration/test_suites/siem_migrations/rules/trial_license_complete_tier/configs/serverless.config.ts

@@ -10,7 +10,7 @@ import { createTestConfig } from '../../../../../config/serverless/config.base';
 
 export default createTestConfig({
   kbnTestServerArgs: [
-    `--xpack.securitySolution.enableExperimental=${JSON.stringify([])}`, // override to empty array so the flag is not disabled
+    `--xpack.securitySolution.enableExperimental=${JSON.stringify([])}`,
     `--xpack.securitySolutionServerless.productTypes=${JSON.stringify([
       { product_line: 'security', product_tier: 'complete' },
       { product_line: 'endpoint', product_tier: 'complete' },