Automatically check for new IntelliJ stable releases

[PawelLipski]

Follow-up for #794.

Should check two things:

• new minor version for the already known major version (like, 2022.1.1 once we already set latestStable to 2022.1): should update latestMinorsOfOldSupportedMajors or latestStable
• first stable release of a new major version: should clean up eapOfLatestSupportedMajor (which will be typically present), update latestStable and append the former latestStable to latestMinorsOfOldSupportedMajors

I'd actually consider making the entire algorithm quasi-stateless, so that:

• latestMinorsOfOldSupportedMajors
• latestStable
• eapOfLatestSupportedMajor

are computed based solely on:

• earliestSupportedMajor
• contents of snapshot repo
• contents of releases repo

and NOT on the other existing properties.

This will make dealing with corner cases less painful in general.

[PawelLipski]

As a part of this task, I'd also update the docs in CONTRIBUTING.md (as they imply all the version updates are to be performed manually, which is/will be no longer the case)

[PawelLipski]

Given that IntelliJ 2022.2 is out, let's queue this one as the next thing after Gradle migration to Kotlin is done (i.e. after #1004)