Fix regression in generic errorbar plot function

Apparently, the handling of left-over dimensions in DataArray.groupby changed such that size-1 dimensions are not necessarily dropped. With this MR, it is made sure that they are squeezed out.

Can this MR be accepted?

• Implementation ready
• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Tested & approved by @lschm

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