JetBrains: Bump min version to 2020.2

Created by: philipp-spiess

I stumble across this property. 2020.2 was the first version to include JCEF by default so we should use this at a minimum now:

https://plugins.jetbrains.com/docs/intellij/jcef.html

Test plan

α jetbrains (ps/jetbrains-bump-min-version) ./gradlew list

> Configure project :
[gradle-intellij-plugin :Sourcegraph] Gradle IntelliJ Plugin is outdated: 1.5.3. Update `org.jetbrains.intellij` to: 1.6.0

> Task :listProductsReleases
IC-2022.1.3
IC-2021.3.3
IC-2021.2.4
IC-2021.1.3
IC-2020.3.4
IC-2020.2.4
BUILD SUCCESSFUL in 1s
1 actionable task: 1 executed

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