Gradle Plugin: Switch extension to a (safe) interface

[hfhbd]

Instead of manually creating the Property via ObjectFactory, you should use an abstract member.

Also, Gradle prefers a safe interface, this will be enforced (with opt-out) via the upcoming Declarative DSL, but it is nice to enforce it as a best practice now too.

Safe means:

• Implemented as an interface
• Does not have a separate implementation type
• Does not have injected services or contain nested injected services

gradle/gradle#35926 (comment)

[drewhamilton]

Nice, thanks. If there's no pressing need, I'll hold onto this until another breaking change has to be made. This will be for Kotlin 2.4.0 at latest.

[hfhbd]

There is no pressing need.