This commit changes the MiMa tests to test against the released 1.4.0 artifacts rather than 1.4.0-rc4; this change is necessary to fix a Jenkins build break since it seems that the RC4 snapshot is no longer available via Maven.

I also enabled MiMa checks for the `launcher` subproject, which we should have done right after 1.4.0 was released.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #6974 from JoshRosen/mima-hotfix and squashes the following commits:

4b4175a [Josh Rosen] [HOTFIX] [BUILD] Fix MiMa checks in master branch; enable MiMa for launcher project