Gradle 8.8 build

[Symeon94]

This adapts the build for the 8.8 version of gradle.
Many deprecated elements were removed in version 8 of gradle.

The publish mechanism might be broken with my current changes. It will need some testing before it can be merged.

[TomasMikula]

Thank you, @Symeon94, for your effort to bring ReactFX to modern Java and Gradle. Given the latest commit in this repo is from 2016, Gradle version used was probably 3.x. No wonder the build has rotted since then.

JReleaser seems to be the currently recommended Gradle plugin for publishing. Gonna have to try it out and see if I can make it work. (It's too late for me today, though.)

[Symeon94]

You're welcome.
While you are there, I'll take this opportunity to ask: I have been told yesterday that this project is no longer being maintained (while it's still being used). If that is the case, is there a way to get information on how we could still keep it up to date (for instance the version pattern used and how we can push changes and perform builds)?

[TomasMikula]

I have published a new version 2.0-M6 using JReleaser. The relevant changes from this PR are squashed in the commit to master.

Regarding maintenance, I can merge an occasional PR like this one that brings the project up-to-date with the tools. Unfortunately, I won't have capacity to evolve the project, like add new features or review PRs with new features. I say that with some sadness, as ReactFX used to be a favorite project of mine.
If you'd like to evolve ReactFX, it'd be best to work on a fork. If it gathers momentum, I'd be willing to let you publish artifacts to the org.reactfx namespace.

[Symeon94]

Thanks for adapting and pushing the content of the build PR. I have pulled your changes on this PR and I'll close it, but I have a follow-up comment on what you have mentioned.
I'm fine working on a fork, but if I was to need a build for use in another project (I'm thinking about RichTextFX, which is my initial drive to update ReactFX), I guess it would be impossible for me to create one (and, subsequently, use the library). What do you advise me to do for that case? (push on a secondary repository under a different name?)