Add 0.23.0 to release schedule (#260)

Signed-off-by: Robert Young <robertyoungnz@gmail.com>

Author: robobario

_data/release-schedule.yml

@@ -5,3 +5,6 @@ releases:
   - version: 0.22.0
     plannedDate: July 3, 2026
     milestoneUrl: https://github.com/kroxylicious/kroxylicious/milestone/10
+  - version: 0.23.0
+    plannedDate: August 20, 2026
+    milestoneUrl: https://github.com/kroxylicious/kroxylicious/milestone/28