Add upgrade notes for the debian repo signing key expiry change

docs/upgrade.md

@@ -137,6 +137,24 @@ room_list_publication_rules:
 
 [`room_list_publication_rules`]: usage/configuration/config_documentation.md#room_list_publication_rules
 
+## Change of signing key expiry date for the Debian/Ubuntu package repository
+
+Administrators using the Debian/Ubuntu packages from `packages.matrix.org`,
+please be aware that we have recently updated the expiry date on the repository's GPG signing key,
+but this change must be imported into your keyring.
+
+If you have the `matrix-org-archive-keyring` package installed and it updates before the current key expires, this should
+happen automatically.
+
+Otherwise, if you see an error similar to `The following signatures were invalid: EXPKEYSIG F473DD4473365DE1`, you
+will need to get a fresh copy of the keys. You can do so with:
+
+```sh
+sudo wget -O /usr/share/keyrings/matrix-org-archive-keyring.gpg https://packages.matrix.org/debian/matrix-org-archive-keyring.gpg
+```
+
+The old version of the key will expire on `2025-03-15`.
+
 # Upgrading to v1.122.0
 
 ## Dropping support for PostgreSQL 11 and 12