JetBrains: Remove usage of APIs added after 2021.1

Created by: philipp-spiess

Closes #37475 (closed)

This still needs testing. I was not able to run on macOS versions prior to 2022.1 because of the JCEF bundling issue we were seeing at the very beginning of our trial. Will verify on Windows instead.

The changes:

• JBCefBrowserBase → JBCefBrowser
• AnActionLink → LinkLabel
• SourcegraphJBCefBrowser#setPageBackgroundColor → removed

Current support

With these changes, I was able to verify support for these versions:

IC-2022.1.3 ✅
IC-2021.3.3 ✅
IC-2021.2.4 ✅
IC-2021.1.3 ✅ 
IC-2020.3.4 ❌
IC-2020.2.4 ❌

2020.2 and 2020.3 are limited by this issue.

Decision:

We decided to drop support for 2020.2 and 2020.3

Test plan

I ran all of these versions on our Debian test machine. Here's an example of a working version (2021.1):

α jetbrains (ps/jetbrains-2020.2-support) ./gradlew listProductsReleases
Starting a Gradle Daemon (subsequent builds will be faster)

> Task :listProductsReleases
IC-2022.1.3
IC-2021.3.3
IC-2021.2.4
IC-2021.1.3
BUILD SUCCESSFUL in 6s
1 actionable task: 1 executed

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.