Previously, unlike the application plugin's run task, our bootRun task
ignored the project's Java toolchain. This meant that the application
was run on a JVM with the same Java version as the one being used by
Gradle itself. This could result in a failure if the application
required a more modern JVM.

This commit updates the plugin to configure the bootRun task's
JavaLauncher convention to be one derived from the project's Java
toolchain. Toolchain support was introduced in Gradle 6.7 so this is
only done when using Gradle 6.7 and later.

Fixes gh-24517