Gradle changes

[dejan2609]

details:

• Gradle related changes: version upgrade: 5.2.1 -->> 7.3.1 plugin 'org.jetbrains.intellij' version upgrade: 0.4.16 -->> 0.7.3 configuration changed from 'testCompile' to 'testImplementation' (https://docs.gradle.org/current/userguide/upgrading_version_6.html) gradle distribution checksum (https://gradle.org/release-checksums) property is added to 'gradle-wrapper.properties' file
• project encoding changed to UTF-8 (in order to solve issues with gradlew file in IntelliJ Idea)
• .gitignore expanded (Gradle, IntelliJ Idea and Eclipse entries are added)
• 'README.md' file is changed (added manual for future gradle upgrades)

[dejan2609]

Please review this @guymahieu

[maartenc]

Thanks for your contribution and sorry for not picking it up earlier. In the meantime, we've upgraded to gradle 8.9 and aligned with the IntelliJ plugin template.