api_changes_list_2021.md: add note about Java 11 requirements
api_changes_list_2021.md: add note about Java 11 requirements

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