Remove deprecated indicators/indices, handle warnings

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes discharge_distribution_fit has wrong _registry_id #1281
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Deprecated Indicators have been removed:
  • first_day_below, first_day_above, snow_cover_duration, continuous_snow_cover_start, continuous_snow_cover_end
• Deprecated indices have been removed:
  • snow_cover_duration, continuous_snow_cover_start, continuous_snow_cover_end
• Most remaining deprecated calls to tests have been updated to new standards
• Indices that emit FutureWarnings through regular use have been identified:
  • snowfall_approximation (method='brown')
  • effective_growing_degree_days
  • See Fix warnings issued from successive convert_units_to calls #1319

Does this PR introduce a breaking change?

Yes. This removes all currently-marked deprecated indices and indicators.

Other information:

The volume of warnings emitted from pytest is significantly reduced. The only warnings now emitted are those coming from lower-level libraries, like numpy and xarray. This should make it easier to identify and handle these instances moving forward.

[aulemahal]

I realized that we are emitting DeprecationWarning instead of FutureWarning. As we discovered elsewhere, the former is not visible to the user by default... Which means, that all the deprecation we are removing here have not been properly announced to the users...

Is the HISTORY enough to justify removing them? Or do we want to wait one more version ?

At least, I think we should fix the issue here :

xclim/xclim/core/indicator.py

Lines 1351 to 1357 in e0f67df

	def _show_deprecation_warning(self):
	warnings.warn(
	f"`{self.title}` is deprecated as of xclim v{self._version_deprecated}. "
	f"See the xclim release notes for more information.",
	DeprecationWarning,
	stacklevel=3,
	)

[Zeitsperre]

@aulemahal

I think we would be fine effecting these changes. In some cases, the history has indicated that we are removing these functions as far back as v0.39, however, if we opt to wait until v0.43, I could remove all remaining deprecated indicators/indices as well.

If we want to mark this PR for v0.43, I'd open another PR to deal with the FutureWarning issue, and maybe address the usage warnings I found in the testing suite. What do you think?

[aulemahal]

✝️ 🎤 💦
Ite in pace, glorificando vita vestra Dominum
Amen.