follow element-web release tagging scheme incl RCs etc

#### Is your feature request related to a problem? Please describe.
Confusing release tagging scheme, see https://github.com/vector-im/element-android/issues/3343#issuecomment-842525636

#### Describe the solution you'd like.
I suggest element-android follow the release tagging style of element-web and most matrix-org repos of tagging RCs explicitly and keeping them. It would be a good look for element to be consistent over its own repos.