Previously, our conventions configured the source and target
compatibility on the project's JavaCompile tasks. This causes the
settings to be missed when importing the projects into Eclipse [1].

This commit updates the conventions to set the source and target
compatibility on the project-wide JavaPluginExtension. This allows
the Eclipse import to correctly detect and honour the settings.

Closes gh-26932

[1] https://github.com/eclipse/buildship/issues/978