[ENH]: Make rotation_mode="xtick" the default for ticks #29926

#28968 introduced new rotation modes "xtick"/"ytick". We've employed them to all our examples, as this rotation setting is superior compared to fiddling with horizontal/verticalalignment. It's "how tick rotation should be".

Proposed solution

Make these the default rotation modes for tick labels.

Technically, this is a breaking change, but I argue it's the better default and the breakage is minor:

• If users did not specify alignment, it's changed from "centered" alignment to the "pointing to tick" aligment.
• If users have cared to do the "pointing" to tick alignment manually (which is what we were teaching in examples before), their plot will stay the same.
• If users have chosen other alignment, they'll need to adapt.

This is a tradeoff between making a reasonable default so that in the future rotated ticks are "just right" and people don't have to do rotation_mode="xtick" explicitly all the time and breaking explicit alignment compatibility for a few rare edge cases.