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The full meaning is hard to tell as you left out half of the error - the part after "but".
But just talking about this displayed Java version, it means that where you consume `convention-plugins` you use Java 11 to run the build (toolchains do not matter here, as it is about a plugin you want to apply, so the version you run Gradle with is the relevant part, toolchains are just what is built).
And if the Java version is, what is complained about, then it means that `convention-plugins` was built with Java 12 or newer and not with a Java 8 toolchain if that is what you meant.

oh. so, plugins are different or don't have a mechanism for toolchains, for their own plugin-developing purposes?

i was hoping to achieve more jdk/toolchain independence, but the plugin level seems to add that hurdle back in, i guess.

For developing a plugin you can of course use toolchains, as I said.
But not for consuming, because you consume them in the Gradle build and thus the Java you run Gradle with is used.