Develop major release

This was already discussed after NWERC and I think it makes sense given the "major" changes and new scoring implementations.

Author: vmcj

ChangeLog

@@ -1,6 +1,6 @@
 DOMjudge Programming Contest Judging System
 
-Version 8.4.0DEV
+Version 9.0.0DEV
 ---------------------------
  - Get rid of 'internal' data source mode, always requiring - but auto
    generating - external ID's for all entities to simplify event logic.

README.md

@@ -6,7 +6,7 @@ DOMjudge
 [![Coverity Scan Status](https://img.shields.io/coverity/scan/671.svg)](https://scan.coverity.com/projects/domjudge)
 [![CodeQL alerts](https://github.com/DOMjudge/domjudge/actions/workflows/codeql-analysis.yml/badge.svg?branch=main&event=push)](https://github.com/DOMjudge/domjudge/actions/workflows/codeql-analysis.yml)
 
-This is the Programming Contest Jury System "DOMjudge" version 8.4.0DEV
+This is the Programming Contest Jury System "DOMjudge" version 9.0.0DEV
 
 DOMjudge is a system for running a programming contest, like the
 ICPC regional and world championship programming contests.