Fixes pure Java plugins buildscripts

[andsel]

Release notes

[rn:skip]

What does this PR do?

With PR #14952 the version of libraries used in Logstash Gradle's buildscript was centralized. However this causes problem in building pure Java plugin, exposed in #14963. The plugin's Gradle script apply the Logstash's core rubyUtils.groovy which needs the definition for the versions. Before apply only plugins and buildscript keywords can be used and there isn't an easy way to share buildscript's pluging version in projects that shares in such way unless we create published plugin.

Why is it important/What is the impact to the user?

As a Logstash pure Java plugin developer I don't want to specify some versions of dependencies used internally by the buildscript.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• test with plugin
• [ ]

How to test this PR locally

• from a local clone of https://github.com/logstash-plugins/logstash-input-java_input_example/
• set the property LOGSTASH_CORE_PATH in your gradle.properties pointing to a checkout of this PR.
• run any ./gradlew task in the plugin, it must success.

Related issues

• Fixes Problem building plugin #14963
• Relates Centralized version definition for snakeyaml and shadow plugin used in buildscripts #14952

Use cases

Screenshots

Logs

[robbavey]

@andsel Is this change still relevant/worthwhile?

[andsel]

Depends on how central are considered the pure Java plugins, in such case the answer is yes.

[mergify]

This pull request does not have a backport label. Could you fix it @andsel? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.