prerequisites.md: note about broken Gradle 6.1
prerequisites.md: note about broken Gradle 6.1

artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs