[MNT]: Unite property cycles for lines and patches

Nobody has claimed this yet.

### Summary

Loosely insprired by #19479.

We have separate property cycles for lines and patches. This is a source of confusion and IMHO doesn't give a benefit.

### Proposed fix

Unite the property cycles so that an Axes has just one cycle to draw from, no matter what the Artist is.

- Most simple plots use only either lines or patches. They wouldn't know the difference.
- If plots have both lines and patches, e.g. one line, one bar plot, IMHO it's more clear to by default use different colors.
- It's quite unlikely that people implicitly rely on synced colors to intentionally have parallelism between the lines and colors.

Migration:
- Add/document a way to get the state of the next element in *the* cycle (as long as there are two, use the line cycle). This will enable explicit use of a cycle element for multiple artists like this:
```
prop = ax.prop_cycle_next()
ax.plot(..., color=prop['color'])
ax.bar(... color=prop['color'])
```
- Maybe consider ways to feed a cycle element back into a plotting function, either through an explicit kwarg `ax.plot(..., prop=prop)`, or by ensuring that the cycle element can be dict-unpacked `ax.plot(..., **prop)`.
- Minimal announce this as upcoming change in the release notes. If people are relying on parallel cycles they should move to the above pattern.
- We could also detect when both cyclers are used and issue a warning. The downside is that people will get a warning as soon as they use a mixture of lines and patches and they may be ok with the new behavior (it's unlikely they really intend to use the old behavior).

Start by reading the issue and the related discussion in #19479, then locate Matplotlib's separate property-cycle handling for lines and patches. Define how one shared cycle and explicit reuse of a cycle element should work, including migration and release-note implications; the unresolved API alternatives indicate design work before implementation.

Written by the indexing model from the issue text.