release: unify versioning, add previousRelease and upcomingRelease

Created by: bobheadxi

• Replaces majorVersion, minorVersion in configuration with previousRelease and upcomingRelease
  • This is required for https://github.com/sourcegraph/sourcegraph/issues/15074 , where some of the formatting depends on what the previous release was (e.g. headers with RELEASE_A -> RELEASE_B). Opens potential for other types of automation as well.
• Removes all version arguments - versions are already defined in config so there should be no need to have this defined as arguments as well. Simplifies release template.

Updated release process: https://github.com/sourcegraph/about/pull/1880