xml: rng: Keep score-attribute{,-mangle} in the existing RelaxNG schema files for graceful upgrades

This commit partially reverts commit
30383cc5d70d542a95860a043f4d044f9fa5af5e to prevent either rolling
upgrade or offline upgrade from being possibly broken.

So that the removal of the attributes score-attribute and
score-attribute-mangle only remains for constraints-next.rng as a
reminder. We'll remove the attributes in the next bumped RNG version and
create a XSL file for doing transformation.