Partially refactor to use more idiomatic Gradle

[3flex]

The main changes are:

1. Use managed properties, where possible
2. Avoid passing instances of Project around
3. Use the plugin to set default values in tasks and the extension

Parts 1 & 3 are not fully completed but I didn't want to press on if this wasn't desired. This should be safe to merge as is though as compat tests for Gradle 6 on Java 11 pass locally.

[3flex]

Looks like failure is due to #228

[TWiStErRob]

Nice one! I'm curious too, @szpak, if this is a desired changeset to continue working on? Probably the best time for 2.0. I would be happy to help on this.

[TWiStErRob]

@3flex re #228, probably temporarily ignore the test that fails and observe green CI without them, just to be sure and then revert.

[szpak]

Nice one! I'm curious too, @szpak, if this is a desired changeset to continue working on? Probably the best time for 2.0. I would be happy to help on this.

I have to admit, I don't closely follow the recent changes in Gradle and how the "idiomatic Gradle" looks like in 2023 :-/. However, in general those changes look fine as simplifications and especially with the already mentioned Robert's branch, they would be nicely fit in the upcoming 2.0.0.

[TWiStErRob]

I think this is good as it is and can be merged, I'm happy to pick up and rebase my branch on top. I can also fix #241 (comment) there.

[szpak]

Thanks @3flex for the PR and @TWiStErRob for the code review!

I think this is good as it is and can be merged, I'm happy to pick up and rebase my branch on top. I can also fix #241 (comment) there.

Sound good.