2.2.20 release preparations

[akarnokd]

Version 2.2.20 - October 6, 2020 (Maven)

JavaDocs

⚠️ The 2.x version line is now in maintenance mode and will be supported only through bugfixes until February 28, 2021. No new features, behavior changes or documentation adjustments will be accepted or applied to 2.x. It is recommended to migrate to 3.x within this time period.

Bugfixes

• Fix Observable.flatMap with maxConcurrency hangs (2.x: Fix Observable.flatMap with maxConcurrency hangs (#6947) #6960)
• Fix Observable.toFlowable(ERROR) not cancelling upon MissingBackpressureException (2.x: Fix toFlowable(ERROR) not cancelling upon MBE #7084)
• Fix Flowable.concatMap backpressure with scalars. (2.x: Fix Flowable.concatMap backpressure w/ scalars #7091)

[akarnokd]

Closing via https://github.com/ReactiveX/RxJava/releases/tag/v2.2.20