Update plugin versioning policy

[gasparnagy]

This PR specifies our versioning policy and updates the versioning policy for Reqnroll plugins.

Also changes the dependency specification of the plugins accordingly.

Types of changes

• [x] Refactoring (so no functional change)

Checklist:

• [ ] I have added an entry to the changelog. (mandatory)
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.