[iterator.concept.winc] `I` used to represent both an integer-class and weakly_incrementable type

The first 2 exposition-only concepts, is-integer-like and is-signed-integer-like, have a template parameter named T.

Then comes the weakly_incrementable concept, with a template parameter named I.

The following normative paragraphs use I to define "integer-class", a subset of the T above. They rightfully reintroduce I each time, although the first one is unused.

Then, the final normative paragraph (before the Note: and Recommended practice:) uses I again, but this time to describe weakly_incrementable.

I is widely used throughout [iterator] to represent iterators. I believe those describing integer-class types should be renamed to T. This would be consistent with the is-integer-like concept.

Related: #3457.