3.0.8-RC1 release preparation

[akarnokd]

Version 3.0.8-RC1 - December 2, 2020 (Maven)

JavaDocs

This is a pre-release for 3.0.8 to verify the release process still works after the switch to GitHub actions (#7114).

Bugfixes

• Remove unnecessary cancel/dispose calls from terminating using (3.x: Remove unnecessary cancel/dispose calls from terminating using #7121)

Documentation

• Flowable scan/scanWith backpressure documentation update (3.x: Flowable scan/scanWith backpressure documentation update #7110)

[akarnokd]

Closing via https://github.com/ReactiveX/RxJava/releases/tag/v3.0.8

BinTray -> Sonatype sync is quite hectic today. The bintray plugin also hangs for some reason.