Replace hacks that worked on old versions of xarray with proper xarray calls

[mikapfl]

Pull request

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Description in a .rst file in the directory changelog_unreleased added - remember to include a * to make it a bullet point

Description

The latest (not-yet-released) version of xarray contains a rework of indexing and some small changes to our I/O functions are necessary to support both old and new xarray.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.26%. Comparing base (472f94d) to head (7f06ffc).
Report is 563 commits behind head on main.

Files	Patch %	Lines
primap2/pm2io/_interchange_format.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
- Coverage   97.30%   97.26%   -0.04%     
==========================================
  Files          32       32              
  Lines        2889     2891       +2     
==========================================
+ Hits         2811     2812       +1     
- Misses         78       79       +1     

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