Deprecate the opacity argument for plot3d and use alpha

[mathzeta2]

Most 3d plot functions use the "opacity" keyword argument to set the transparency of the plot. On the other hand, most 2d plot functions use the "alpha" keyword argument.

It seems that the "opacity" keyword argument is also not well-documented. It sometimes appears only in the Examples sections, and not in the Input section. This is the case, at least in the following:

• plot3d()
• line()
• circle()
• parametric_plot3d()
• revolution_plot3d()

Harmonizing the 2d plot and 3d plot functions would be a better UI. For example, changing a single letter of obj.plot2d(some_args, alpha=0.5) to obj.plot3d(some_args, alpha=0.5) (or vice versa) should "just work", without ignoring arguments.

[yzh]

See #25095. In the plot3d functions, the argument "alpha" is equivalent to "opacity" now.