[Feature] Migrate to Gradle 7.6

[DissonantAU]

Upgrade Gradle to 7.6

Some Tweaks to the Packaging Plugin will be needed, but it can better support Kotlin and work better with newer versions of JDK .
Better support for Compiling to different JRE Targets can be added at the same time.

[DissonantAU]

I've forked the project, and created a branch based on #75: (develop branch plus fixes)
https://github.com/DissonantAU/TouchPortalPluginSDK/tree/develop-gradle-7.6

• Updated to Gradle Wrapper to 7.6.4
  • Several changes made to allow builds to work, including mavenToLocal
  • Also builds as an includedBuild in projects using Gradle 8.5
  • Signing only happens if 'public' task is in dept graph
• Packager Plugin Compatibility Fixes and
  • Can now be given an Integer or provider to set a JRE Release target (8 by default)
  • Can build SDK with newer JDK (e.g. 17) and target JRE 8, 17, etc.
  • Now uses kotlin-gradle-plugin library

I'm also still using SDK 8.3 for some plugins, so I also forked the master branch, dropped the 9.3 merge, and applied the same updates: https://github.com/DissonantAU/TouchPortalPluginSDK/tree/dev-8.3

I also backported Dependency Resolution Management and some other changes for builds.
It'd be great if 8.3 could be supported for a while longer, with updated releases here and on Maven, so people using it with their projects don't have to update to 9.0 just yet.

(Edit: Clean-up after fixes in #75 branch)

[DissonantAU]

Note: Rebased onto #75