Add 22.04 note on deprecations and metadata changes

docs/installation-and-operations/operation/upgrading/README.md

@@ -33,6 +33,22 @@ sudo apt-get install --only-upgrade openproject
 sudo openproject configure
 ```
 
+**A note for Ubuntu 22.04 installations**
+
+On Ubuntu 22.04., you might see warnings like these:
+
+> W: https://dl.packager.io/srv/deb/opf/openproject/stable/12/ubuntu/dists/22.04/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
+
+This message is due to Ubuntu 22.04 switching to a more secure way of adding repository sources, which is not yet supported by the repository provider. There is ongoing work on this item, the message is for information only.
+
+If you get an error like the following:
+
+> E: Repository 'https://dl.packager.io/srv/deb/opf/openproject/stable/12/ubuntu 22.04 InRelease' changed its 'Origin' value from '' to 'https://packager.io/gh/opf/openproject'
+> E: Repository 'https://dl.packager.io/srv/deb/opf/openproject/stable/12/ubuntu 22.04 InRelease' changed its 'Label' value from '' to 'Ubuntu 22.04 packages for opf/openproject'
+
+These two messages messages are expected, due to a change in Origin and Label repository metadata, to better explain what the repository is about. You should allow the change, and/or run `sudo apt-get update --allow-releaseinfo-change` for the update to go through.
+
+
 ### CentOS / RHEL
 
 ```bash