[Serverless release notes] Updates known issues page structure (#1451)

This PR does the following:
- Updates the known issue summary for the Elastic Defend bug that
reverted rule customizations when a new integration policy was created.
The summary now shows that the bug is resolved.
- Splits the Serverless known issues page into two categories: active
and resolved bugs. This change allows users, support, and other
important stakeholders to quickly understand whether known issues have
been fixed or are still active in Serverless.

Preview: [Elastic Cloud Serverless known
issues](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1451/release-notes/elastic-cloud-serverless/known-issues)

Author: nastasha-solomon

release-notes/elastic-cloud-serverless/known-issues.md

@@ -14,7 +14,11 @@ Known issues are significant defects or limitations that may impact your impleme
 % **Workaround** 
 % Workaround description.
 
-:::
+## Active
+
+There are no active known issues. 
+
+## Resolved
 
 :::{dropdown} In {{sec-serverless}}, installing an {{elastic-defend}} integration or a new agent policy upgrades installed prebuilt rules, reverting user customizations and overwriting user-added actions and exceptions
 
@@ -24,4 +28,8 @@ On April 10, 2025, it was discovered that when you install a new {{elastic-defen
 
 To resolve this issue, before you add an {{elastic-defend}} integration to a policy in {{fleet}}, apply any pending prebuilt rule updates. This will prevent rule actions, exceptions, and customizations from being overwritten.
 
+**Resolved**
+
+This was resolved on April 14, 2025.
+
 :::