Creating a release process based on GH Actions

[twasyl]

The objective was to create a release process based on GitHub Actions and no more use travis-ci and AppVeyor. The use of Bintray is also discarded. To achieve this:

• a task updateProductVersionNumber for updating the @@NEXT-VERSION@@ token in files has been created;
• a task removeSnapshots removing the -SNPASHOT qualifier in gradle files has been created;
• a .github/workflows/main.yml workflow has been updated to work on branch and tags. A GitHub release is created when a tag is pushed;
• the coverage has been made working properly with unit, integration and Gherkn tests;
• the SonarQube analysis is performed;
• the custom SFXPublisher gradle plugin has been removed as it uploaded the artifacts to Bintray;
• release notes are in dedicated files by version. This ease the creation of the GitHub release.

Along with that, some other changes have been made:

• gradle has been upgraded to 6.5;
• Some dependencies have been upgraded;
• The code extension adds new supported languages.