Do not set JDK 1.8

[osipxd]

Setting of JDK 1.8 should be removed. JDK should be set by developer using JVM Toolchains instead.

BukkitGradle/src/main/kotlin/BukkitGradlePlugin.kt

Lines 39 to 42 in 8ab3db4

	extensions.configure<JavaPluginExtension> {
	sourceCompatibility = JavaVersion.VERSION_1_8
	targetCompatibility = JavaVersion.VERSION_1_8
	}

Causes #60

[osipxd]

I can set defaults for JVM toolchain and use Spigot requirements to select the recommended version.