Applying changes from the Spring Academy port

[cschuyle]

These are the changes we've made in the Spring Academy incarnation of this Guide ... but only the changes that are not specific to "in-Cloud" (Spring Academy) mode (i.e. instructions that differ for using the Educates workshop and its in-browser UI to follow the instructions, instead of the learner's local machine).

This PR is a placeholder for a conversation (or more than one).

Summary of changes:

• Removed the codebase (initial and complete directories) entirely and adjust the instructions to start from an Initializr-generated codebase.
  • Because of this, didn't use the existing include files:
    • build_an_executable_jar_mainhead.adoc
    • build_an_executable_jar_with_both.adoc
• Removed the maven option - all instructions only reference gradle.
• Updated terminal output to match Spring Boot 3.1.3
• Moved the material from the "Spring Boot Starters" section, to the first mention of them (the "Add Tests" section)
• Added two explicit sections for the tests material: "Unit Test" and "Integration Test"
• Removed Groovy material - the spring CLI doesn't do that anymore.
• Section currently called "JAR support and Groovy support" is now called "Build an executable JAR or WAR"
• Added reference to the Building a REST API with Spring Boot and Spring Boot courses in the conclusion, after the references to the guides.

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[Buzzardo]

I've added some review comments to the code.

In addition to those:

Removed the codebase (initial and complete directories) entirely and adjust the instructions to start from an Initializr-generated codebase.

I'm not sure if we should do this or not, but if we do I think we need to review all guides to make sure that we are consistent.

Removed the maven option - all instructions only reference gradle.

I'm not sure that we should do this. Although Gradle is the default choice on start.spring.io many folks still use Maven.

I very much agree. First, we have 60 or so guides. That's a lot of change to match to only one changed guide.

Also, the Spring team itself uses both Gradle and Maven, and there are lot of strong feelings for and against both of them. As a case in point, Spring Batch moved from Gradle to Maven in the latest major version (version 5). Since the advent of Gradle, we have never been able to form a consensus about which to use.

On top of the internal issue, the Spring community uses both Maven and Gradle (sometimes in the same shop, just as we do), and I'd hate to lose either one.

[cschuyle]

Thanks for all your comments, folks! I've incorporated some into the current spring-guides repo.

[cschuyle]

Closing this PR because we've de-prioritized this effort, and recognizing that there are still at least two main points of conversation to pick up when the effort is re-kindled:

• Handle both maven and gradle in every guide
• Remove the codebase initial and complete, if the initial is actually handled by spring initializr.