Optimize Gradle build on GitHub

[ale5000-git]

I have done some changes:

• Added "workflow_dispatch", this will allow you to run the build manually if needed in addition of the automatic run;
• Remove all permissions from the actions since they aren't needed for a simple build;
• Update the versions of actions (older versions will probably break soon);
• Start using the official action to setup Gradle since it handle caching automatically in a better way compared to setup java action (the first run will be slower due to cache changes but the following runs will decrease);
• Remove useless ENV variables, the ram of Gradle is already configured inside gradle.properties and the other is probably a remnant of the past;
• Minor cosmetic improvements.