Use Gradle 7.6 to build Gradle plugin

Created by: erichaagdev

This PR updates the openapi-generator-gradle-plugin project to build with Gradle 7.6, the latest.

All plugin versions have been updated to their latest versions. The plugins io.codearte.nexus-staging and de.marcphilipp.nexus-publish have been replaced by io.github.gradle-nexus.publish-plugin as recommended by the original plugin authors. Aside from the configuration, the only notable difference is that "Nexus" has been replaced with "Sonatype" in the names of the relevant publish tasks.

I'm not sure why previously there existed both a mavenJava and pluginMaven publication. For a Gradle plugin, the only publication needed is pluginMaven created by the java-gradle-plugin plugin. I've removed the mavenJava publication. Only pluginMaven is published to Sonatype.

I am unable to test publishing to Sonatype. However, I compared the before and after output of publishToMavenLocal and saw no notable differences.

I did not update the sample project as it does not depend on the version of Gradle used to build the plugin itself. I also did not want to make this PR any larger.

Gradle Build Scan: https://gradle.com/s/d4jzctozh7gem Gradle 7.6 Release Notes: https://docs.gradle.org/7.6/release-notes.html

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.1.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.