[chore] Removes version from contributing.md

[gruyaume]

Description

The goal here is to make our life easier when needing to update the Vault version. Now we only have to update 1 file when that's the case (rockcraft.yaml)

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that validate the behaviour of the software.
• I validated that new and existing tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.