Previously, our Gradle plugin was not tested against Gradle 6.0,
a number of deprecation warnings were output when using the plugin
with Gradle 6, and some functionality related to the application
plugin did not work as expected.

This commit tests the plugin against Gradle 6. It also avoids calling
deprecated APIs. The plugin is compatibile against Gradle 4.10 where
the deprecated APIs' replacements are not available so reflection is
used to call the replcaements. Lastly, the way in which the base name
of the boot distribution that is created when the application plugin
is applied has been modified to ensure that it is effective when using
Gradle 6.

Closes gh-18663