Fix Gradle plugin publishing

[erichaagdev]

Publishing of the openapi-generator-gradle-plugin to Sonatype was broken following #13860 due to an incorrect task name in the Travis CI configuration. This PR replaces the incorrect task name (closeAndReleaseRepository) with the correct one (closeAndReleaseSonatypeStagingRepository).

I have created a Build Scan showing a dry run of the plugin publish following the update to Gradle 7.6: https://scans.gradle.com/s/zwwybxi6jbcam/console-log?page=1#L18

I also created a Build Scan showing a dry run of this process as it was in Gradle 5.6.4, from before my changes were made: https://scans.gradle.com/s/ngvxlsxc4fkyy/console-log?page=1#L15

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 (6.3.0) (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.

[erichaagdev]

cc @wing328, @sorin-florea, @guylabs

[erichaagdev]

FYI, since merging #13860 the lines to publish the Gradle plugin were temporarily removed so that other publications can continue. This is why it looks like I'm adding lines which were not there before.

You can see a diff between the .travis.yml file changes as they were in #13860 and this PR here.