Document unsupported reason 'home_assistant_core_version'

Document when we declare an installation as unsupported due to outdated
Home Assistant Core version.

Author: agners

source/more-info/unsupported/home_assistant_core_version.markdown

@@ -0,0 +1,34 @@
+---
+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 older than two years as unsupported.
+We generally recommend to always update to the latest Home Assistant Core, or
+at least update within 6 months to the latest release to not miss out on
+automatic migration of YAML integration (the migration code to migrate YAML
+integration to regular config entry based integration is available for 6
+months, today this is for 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
+
+Since you've not updated your system in a while, we recommend to [create a backup](/common-tasks/general/#backups)
+before updating your system. Make sure to download the backup or store it at
+a remote location.
+
+To resolve this issue, 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" %}
+enabling skipped updates.
+
+If you would like to take advantage of YAML integration migration, update 6
+releases at a time (e.g. from Home Assistant Core 2024.2 to 2024.8). You can
+use the HA CLI to update to a specific version.