Rename BootRun, BootJar and Boot war's main class property to mainClassName, making them consistent with each other and the application plugin

[wilkinsona]

BootRun uses main whereas BootJar and BootWar use mainClass. Now that BootRun is no longer a JavaExec subclass we can rename the property to mainClass to make things consistent.

[wilkinsona]

I'm now wondering if the property should be renamed to mainClassName. While longer, it would then be consistent with the application plugin's mainClassName project property. The property's also rarely used as, typically, there'll be a single main method and it will be automatically found by MainClassFinder which is used by both the Maven and Gradle plugins.