[Core] Migrate SO _migrate route authorization to the new system

[kyracho]

Summary

Hi! 😊 this PR deals with #198181

router.post({
  path: '/_migrate',
-  options: {
-    tags: ['access:migrateSavedObjects'],
+  security: {
+    authz: {
+      requiredPrivileges: ['migrateSavedObjects'],
+    },
  ...
}, handler);

Checklist

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• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
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• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_node:* label is applied per the guidelines

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• ...

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[jloleysens]

/ci

[jloleysens]

Thanks, LGTM. Will approve and merge once CI is passing!

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: ef4ec14

Metrics [docs]

✅ unchanged

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)

[github-actions]

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