Document unsupported reason 'home_assistant_core_version' (#40914)

Co-authored-by: Copilot <[email protected]>
Co-authored-by: c0ffeeca7 <[email protected]>

Author: 3 people

source/more-info/unsupported/home_assistant_core_version.markdown

@@ -0,0 +1,36 @@
+---
+title: "Home Assistant Core version"
+description: "More information on why Home Assistant Core version marks the installation as unsupported."
+---
+
+## The issue
+
+Supervisor considers Home Assistant Core versions older than 24 months (approximately 24 releases) as unsupported.
+It is strongly recommend to keeping Home Assistant Core updated to the latest version.
+At a minimum, update within 6 months (6 release cycles) to ensure you don't miss
+automatic YAML integration migrations. Migration code that converts YAML-based
+integrations to modern UI-based configuration is only available for 6 months
+after deprecation, which corresponds to 6 Home Assistant Core releases.
+
+On installations using an unsupported Home Assistant Core release, the
+Supervisor stops refreshing its update information. This means you will no
+longer receive updates for any component, including Home Assistant Core or
+Add-on updates.
+
+## The solution
+
+1. Since you've not updated your system in a while, it is recommended to
+   [create a backup](/common-tasks/general/#backups) before updating your system.
+    - Make sure to download the backup or store it in a remote location.
+
+2. Update your Home Assistant Core version.
+    - Go to {% my updates title="**System** > **Updates**" %} to update Home Assistant Core.
+    - If you don't see the update, you may have skipped it in the past. To see
+      previously skipped updates, select the three dots {% icon "mdi:dots-vertical" %}
+      and enable skipped updates.
+
+    - If you're significantly behind and have YAML-based integrations that need
+      migration, update in increments of no more than 6 releases at a time
+      (for example, from Home Assistant Core 2024.2 to 2024.8, then to 2025.2).
+      - This ensures automatic migrations can run properly during each update.
+      - You can use the Home Assistant CLI to update to a [specific Home Assistant Core version](/common-tasks/os/#home-assistant-versions).