Deprecate project; users should go to maven-executor instead

### Affected version

all

### Bug description

The maven-executor replacement should be used that among other things offer smooth and transparent mvn3 and mvn4 support and same if not more functionality as maven-invoker and much more, like proper env isolation and so on.

No files, tests, or entry points are named. Start by determining how this deprecated project should direct users to maven-executor and what deprecation work is expected; done means the project is clearly deprecated and users have actionable guidance.

Written by the indexing model from the issue text.