v0.4.11: Remove dependency on deprecated conventions API (#54)

`Project.convention` and other conventions API methods have been
deprecated and will be removed in Gradle 9.0. To make things worse, the
`Project.convention.plugins["java"]` does not return
`DefaultJavaPluginConvention` anymore, but instead returns a wrapper
class that issues deprecation warnings. Since the plugin cannot find the
expected class, it stopped working properly (this is probably the cause
of #53).

This pull requests simply replaces the single use of
`Project.convention.plugins[]` (finding the Java test sourceset) with
`Project.extensions.findByType`.

Fixes: #42, #53