Older versions of IDE support

[denis-fokin]

Description

We have switched to newer versions of Java/Kotlin/Gradle and upgraded minimal version of IntelliJ platform. It seems that we are still need the older platform support. 2021.2 is a good candidate because it is the supported version by Android Studio.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Automated Testing

Automated CI tests

Manual Scenario

Try to install plugin on 2021.2

Checklist (remove irrelevant options):

• The change followed the style guidelines of the UTBot project
• Self-review of the code is passed
• The change contains enough commentaries, particularly in hard-to-understand areas
• New documentation is provided or existed one is altered
• No new warnings
• New tests have been added
• All tests pass locally with my changes

[onewhl]

Looks good to me

[alisevych]

This MR fixed #808