Add warning about MySQL performance impact to GHES 3.10.2 release notes (#43395)

Author: mattpollard

Diff for: data/release-notes/enterprise-server/3-10/2.yml

@@ -1,4 +1,10 @@
 date: '2023-09-22'
+intro: |
+  {% warning %}
+
+  **Warning**: A change to MySQL in GitHub Enterprise Server 3.9 and later may impact the performance of your instance. Before you upgrade, make sure you've read the "[Known issues](#3.10.2-known-issues)" section of these release notes.
+
+  {% endwarning %}
 sections:
   bugs:
     - On an instance in a high-availability, geo-replication, or repository cache configuration, prolonged replication issues could occur on replica nodes due to failure of `SpokesRepairRepoReplicaJob` and `SpokesSyncCacheReplicaJob` jobs.