Gradle deprecation fixes & upgrades (#1594)

* Fix Gradle deprecations

By using test suites, we explicitely configure the relevant dependencies
on the test sourceset. This is not done by merely configuring the test task.

* Switch to maintained version of Shadow

* Update to Gradle 8.14.2

gradle/libs.versions.toml

@@ -8,7 +8,7 @@ netty = "4.2.1.Final"
 [plugins]
 fill = "io.papermc.fill.gradle:1.0.3"
 indra-publishing = "net.kyori.indra.publishing:2.0.6"
-shadow = "io.github.goooler.shadow:8.1.5"
+shadow = "com.gradleup.shadow:8.3.6"
 spotless = "com.diffplug.spotless:6.25.0"
 
 [libraries]