Make AjcForkOptions abstract and instantiate it via ObjectFactory

[asodja]

For better compatibility with Gradle 9.

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In Gradle 9 we plan to replace task/extension properties with Provider API lazy types (Property/Provider/ConfigurableFileCollection). And with that BaseForkOptions will become abstract and have abstract methods, see change:
https://github.com/gradle/gradle/pull/30374/files#diff-c132a92f383d74b12cf1b727c41b2dbc07fffecb7f1ad19c6b1e343ee75cbd30

During smoke tests we found out that AcjForkOptions extends BaseForkOptions. But with Gradle 9 API it then fails with:

Execution failed for task ':compileAspectj'.
    > Receiver class io.freefair.gradle.plugins.aspectj.AjcForkOptions 
    does not define or inherit an implementation of the resolved method 
    'abstract org.gradle.api.provider.Property getMemoryInitialSize()' of 
    abstract class org.gradle.api.tasks.compile.BaseForkOptions.

A solution to make it compatible with Gradle 9 is to make AjcForkOptions abstract and instantiate it via ObjectFactory. This is also ok for previous Gradle versions.