Grails 5.2.1 upgrade

[tylervz]

Upgraded the example to use Grails 5.2.1 applications. The main changes are:

1. switching from jar to bootJar in complete/build.gradle so that the correct runnable jar file is copied into the Docker image.
2. changing the base Docker image from openjdk:8u151-jdk-alpine to eclipse-temurin:11-jdk-alpine.
3. Because the base Docker image has changed, the working directory for the application's Docker image has changed from /app to /home/app and the entrypoint shell script has been updated.
4. Upgrading the Docker gradle plugin from 6.0.0 to 6.7.0. We could upgrade it to the newest version, 7.4.0, but that would require the user to be using Java 11+.

To get the publishGuide Gradle task working, I had to copy the guide-build.gradle file and update a few things:

1. use "org.grails:grails-docs:5.1.9" instead of "org.grails:grails-docs:3.2.7"
2. Upgrade the plugin from id 'org.asciidoctor.convert' version '1.5.3' to id 'org.asciidoctor.jvm.convert' version '3.3.2'.
3. Change the asciidoctorj version to 2.5.4 from 1.5.4.

I wasn't sure if there was a different branch that I should be applying the guide-build.gradle from which works with Gradle 7.2.