Previously, BootRun used the main property to configure the name of
the main class to run while BootJar and BootWar used the mainClass
property. Both were different to the application plugin which provides
a mainClassName project property.

This commit updates BootRun, BootJar, and BootWar to change the name
of the property used to configure the name of the main class to be
mainClassName. This makes the three Boot-specific tasks consistent
with each other, and also aligns them with Gradle's own application
plugin.

Closes gh-10622