There is some work needed to update the PPA packaging for Pinta 2.0 due to its dependency changes. In particular, instead of mono, dotnet-runtime-6.0 / dotnet-sdk-6.0 are needed which is not in the default Ubuntu repositories, and instead comes from Microsoft's PPA (
https://docs.microsoft.com/en-us/dotnet/core/install/linux-ubuntu). From
https://help.launchpad.net/Packaging/PPA/BuildingASourcePackage#Dependencies, it sounds like it may not even be possible to do that from Launchpad's build system
I'm not an expert on .deb packaging though, so contributions in this area would be very welcome! For the time being though, that message is there so that users are aware that more recent builds can be found elsewhere.