Release Dotty 0.4.0-RC1

Merge pull request scala#3078 from allanrenucci/0.3.x

Backport scala#3076

Release 0.2

Merge pull request scala#3002 from dotty-staging/topic/release-0.3.0-RC1

Change build version number for 0.3.0-RC1 release

Merge pull request scala#2847 from lampepfl/0.2.0-RC1

RLZ 0.2.0-RC1

Merge pull request scala#2619 from dotty-staging/topic/add-release-to-CI

Add release job to CI