More aggressively validate versions.props

[dansanduleac]

What happened?

#372 showcased that we allow multiple ways of defining constraints in versions.props:

• group:name = version
• group:name:version

While the latter worked before, it doesn't get parsed correctly by the new checkUnusedConstraints task, leading to false positives.

What did you want to happen?

Enforce that versions.props only contains constraints of the form group:name = version in order to simplify the logic that reads and validates that file.