This happens because the latest KeePassXC version is 2.4.3. The warning doesn't prevent using the extension. Everything still works normally. If you cannot upgrade and want to get rid of the warning, you can set "Check for updates" from the extension settings to "Never".
However, when looking at the official Arch package, it clearly says 2.4.3-1. See https://www.archlinux.org/packages/community/x86_64/keepassxc/.
EDIT: Looking at the Manjaro Stable repository, the are one release behind, so they only offer 2.4.2.