A Github Action for nightly deploy #3106

On Jenkins, we had a "nightly" build of KeY that was also deployed on the www11 sub-domain at the KIT.

This PR adds a Github Action, which will update (delete and create) a release entry on the Github repo. This entry is marked as pre-release and contains a generated changelog. The current shadow-jar is attached.

Example see here: https://github.com/KeYProject/key/releases/tag/nightly

This allows you to simple download the current version of KeY via gh:

sh $ gh release download nightly -R keyproject/key -p '*-exe.jar'

or by URL

sh wget https://github.com/KeYProject/key/releases/download/nightly/key-2.11.0-exe.jar

Discussion points:

• Add further build artifacts? (Javadoc, ...)

Todo:

• [x] Finalize by restricting the trigger to main branch only
• [x] git rebase -i
• [x] Re-activate other workflows

/cc @mattulbrich @jwiesler @unp1