Previously, the BootRepackage task would take the output of a Jar
or War task and repackage it in a similar manner to Spring Boot's
Maven plugin. This caused several problems in Gradle including
broken up-to-date checks and a lack of configurability. See the issues
referenced below for full details.

This commit replaces BootRepackage with BootJar and BootWar
for building executable jars and wars respectively. BootJar extends
Gradle's standard Jar task and BootWar extends Gradle's standard War
task. This means that terms of configuration, the creation of
executable jars and wars is now as flexible as the creation of
standards jars and wars.

Closes gh-8167
Closes gh-8099
Closes gh-6846
Closes gh-5861
Closes gh-5393
Closes gh-5259
Closes gh-3931