Rename .font-weight-bold to logical state name (.font-weight-strong)

[klasjersevi]

The variable names introduced in #18433 and 968282e for strong text are based on the physical state name bold instead of the logical state name strong. This leads to confusion when you want to change/override these styles with custom styling.

Let's say you want to change the .font-weight-bold style to font-weight: 500; // Medium and then use font-weight: bold; in styles somewhere else, then the names gets confusing.

The names should be changed to match the <strong> element used in html.
Discussions about strong vs bold can be read at MDN - <strong>.

Similar renaming might also be applicable for .font-italic, but thats more open for discussion, since it has another meaning. More to read at MDN - <em>

So I propose renaming to the following:
.font-weight-strong { font-weight: bold; }

[mdo]

I'm not super concerned about the naming of these utilities. If there's enough feedback around it into v4 once it's stable I can add aliases. Until then, no planned changes. Thanks though!