Rework BootRun so that it doesn't subclass JavaExec

[wilkinsona]

Rather than subclassing JavaExec we should subclass DefaultTask and use Project.javaexec(Action<JavaExecSpec>) instead. This will give us precise control over the configuration that's exposed to users and how that configuration is passed to the forked JVM, making #1176, and possibly #3982, easier to implement. It will also allow us to remove all our use of convention mappings which is planned as part of #9516.