Resolve "Remove (deprecated, non-faceting) `generic.errorbar` plot"

This MR removes the deprecated errorbar and errorbands non-faceting plot functions. As the deprecation was already applied in %Version 0.15, this should not be a big disruption now.

How to migrate plots

• Use facet_grid instead and set kind: errorbars
• To use errorbands, set use_bands: true
• The facet_grid function expects an xr.Dataset and the y and yerr arguments:

my_errorbars_plot:
  creator: pyplot
  module: .generic
  plot_func: facet_grid
  kind: errorbars
  use_bands: true

  select:
    # ...
  transform:
    - xr.Dataset:
      - mean: !dag_tag mean
        std: !dag_tag std

  y: mean
  yerr: std

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• 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
  • Reviewed & approved

Related issues

Closes #314 (closed)