Deprecate AffineConstraints constructor and reinit() function that take only one index set.

[bangerth]

Follow-up to #15789 and as discussed there.

In reference to #15375.

[kronbichler]

This is a widely used functionality that is likely to affect many codes. I guess we will use the early deprecation now (if we introduce that within the next few weeks) or after the next release, to avoid too abrupt interface changes.