This adds build caching and build scans.

The changes required disabling scans when using the maven invoker
plugin in order to not cause duplicate build scans when invoking other
maven builds. There is also an empty `.mvn` folder in the
spring-boot-starters project to prevent duplicate build scans as well
since there is no way to pass properties to the maven-javadoc-plugin.

The checkstyle plugin was causing a cache miss with the
`propertyExpansion` because it contains an absolute path. The absolute
path is now ignored and instead the files are added as inputs to the
checkstyle plugin. This only enables the local build cache. The remote
cache is not yet enabled.

On my local machine:

./mvnw clean install build times go from about 30 minutes to about 10 minutes.
./mvnw clean install -Pfull build times go from about 60 minutes to about 13 minutes.

See gh-22089