Upgrade IntelliJ platform template

Created by: philipp-spiess

This PR is an attempt to update our plugin to the latest version of https://github.com/JetBrains/gradle-intellij-plugin so we can iterate quicker on potential future changes.

Upgrade instructions are documented here but are mostly focused on Kotlin instead of our existing Groovy config files.

Learnings so far:

• Setter syntax for Groovy also changed
• We'll need to upgrade the IntelliJ version to at least 2021.3 for M1 compat (https://youtrack.jetbrains.com/issue/CWM-3540#focus=Comments-27-5192513.0-0)