Update Guides to Use Gradle

[rwinch]

Summary

Original report #4283 (comment)

[To-da]

Also:

Using Spring 4.0.x and Gradle

should be replaced with:

Using Spring 5.0.x and Gradle

in all 5.0.x branches.
see: https://docs.spring.io/spring-security/site/docs/5.0.6.BUILD-SNAPSHOT/reference/htmlsingle/#gradle-resolutionStrategy