Change solar_zenith to apparent_zenith when angle is refraction corrected

[RDaxini]

Is your feature request related to a problem? Please describe.
The term solar_zenith is used for both refraction-corrected (apparent) zenith angles and non-refraction-corrected zenith angles. Example functions from irradiance.py: klucher, haydavies, reindl, king, perez, and perez_driesse. Also examples in infinite_sheds.py: _shaded_fraction, get_irradiance_poa, and get_irradiance.

Describe the solution you'd like
Replace solar_zenith with apparent_zenith for refraction-corrected solar zenith angles. Both terms already have nomenclature entries.

Additional context
Incorrect (example):

pvlib-python/pvlib/bifacial/infinite_sheds.py

Lines 417 to 418 in 216386e

	solar_zenith : numeric
	Refraction-corrected solar zenith. [degree]

pvlib-python/pvlib/irradiance.py

Lines 662 to 664 in 216386e

	solar_zenith : numeric
	Apparent (refraction-corrected) zenith angles in decimal
	degrees. ``solar_zenith`` must be >=0 and <=180.

Correct (example):

pvlib-python/pvlib/clearsky.py

Lines 35 to 36 in 216386e

	apparent_zenith : numeric
	Refraction corrected solar zenith angle in degrees.

[kandersolar]

@RDaxini see #1403. Let's continue any discussion there for continuity.