Remove faulty default in MRT and UTCI

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes NA values for UTCI - issue with mean radiant temperature calculations? #1496
• 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?

I believe #1399 changed the meanings of argument stat which made the default option of mean_radiant_temperature incorrect. This PR removes the "average" option and changes the default to 'sunlit'.

Does this PR introduce a breaking change?

Yes, outputs will change. However, I think previous output was incorrect. An announcement has been added to the CHANGES.

Other information:

[github-actions]

Note
It appears that this Pull Request modifies the main.yml workflow.

On inspection, the XCLIM_TESTDATA_BRANCH environment variable is set to the most recent tag (v2023.9.12).

No further action is required.

[Zeitsperre]

LGTM

[ludwiglierhammer]

I think this should be adopted to UTCI as well, isn't it?

[Zeitsperre]

@emsonali

Hi there, we're preparing a new release of xclim no later than tomorrow (24 Oct, 12 PM EST). If you have a minute, please let us know if this PR settles the issue you were experiencing with the MRT index. Thanks!

[emsonali]

Hi @Zeitsperre, thanks for the update. I've tested the fix and have updated @aulemahal on the other thread. I just need him to verify if one of the stat settings is working as expected. Following his response, you can proceed.