Transition feature labels to proper category labels

This MR updates the feature labels in categories.yml. Those labels are used to triage issues with designated labels into appropriate categories, groups and stages automatically. Previously there were a number of listed "feature labels" that were redundant as they were named after the category itself. Those redundant labels have been renamed to be Category scoped labels.

1. I've renamed the feature labels that had a 1-1 mapping with categories to follow the Category:<name> convention.
2. Some feature labels and category label were duplicated, so I've made sure to keep only the category label (and migrated the resources to use it).