Fixes for Pandas v2.0

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes issues raised in Prepare v0.42.0 #1343
• 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?

• In xclim.core.missing.MissingBase.prepare, we transform nanoseconds into days. Pandas' timedelta doesn't support it anymore, but we only need the array, so I passed through numpy directly instead.
• In xclim.sdba.base.Grouper.get_index, we relied on the deprecated DatetimeIndex.dt.week, here I used the suggested DatetimeIndex.dt.isocalendar().week instead.
• Also removed the equals test in the make_criteria unittest, it wasn't necessary and failed because of the changed dim order.
• Noted in changelogs that xclim v0.42.0 will be the last version to explicitly support Python3.8.

Does this PR introduce a breaking change?

It shouldn't. This should be completely compatible with pandas 1.5 and a few earlier releases.

Other information:

#1268