Description property for the chart legend does not override the default aria-label

[MarkMcAmhlaidh]

Bug Description

Current Behaviour

• Legend elements always use default aria-label values regardless of the description property being set.
• Custom accessibility labels, set with the description property are ignored.
• Localisation of legend text is not possible.

Expected Behavior

• The description property should properly override default aria-label values.
  https://vega.github.io/vega-lite/docs/legend.html#properties

Impact

• Accessibility: Users relying on screen readers cannot understand chart legends in their language
• Localisation: Cannot provide localised aria-labels international users

Reproduction Steps

• Create a chart with a legend
• Set the description property on the legend configuration
• Inspect the legend elements on your browser
• You will see that the default values are used instead of the custom description

{
  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
  "data": {"url": "data/barley.json"},
  "mark": "bar",
  "encoding": {
       "x": {"aggregate": "sum", "field": "yield"},
       "y": {"field": "variety"},
       "color": {
          "field": "site", 
          "legend": {
             "description": "test custom label",
          }
       }
  }
}

[domoritz]

Please add an example in the issue. Also, would you be willing to look into a fix?