The keepass2 package in Debian/Ubuntu has not been updated since 2021-01-28. https://tracker.debian.org/pkg/keepass2
For reference: System.Environment.Version docs Mono release notes
Add Mono version map to GetMaxNetFrameworkVersion()
If a copy of KeePass is already running, starting a new version will just active the currently running version even if they are not the same version. I occasionally do this when running KeePass from Visual Studio then wonder why it looks wrong. Then I remember this is what is going on and close the other running version.
KeePass 2.x used to work on Mac via the Mono runtime. However, since macOS went 64-bit only, it is not possible due to https://github.com/mono/mono/issues/6701. There are compatible 3rd-party alternatives listed on the KeePass downloads page.
mono-complete is extra packages on top of mono. so mono --version does not indicate that mono-complete is installed.
Usually sudo apt install mono-complete will fix this. Unless the plugin really does need a newer version of KeePass - only the plugin author/docs can tell you this.
PPA: https://launchpad.net/~jtaylor/+archive/ubuntu/keepass