glossary: move labels to label (#7486)

The term_id for Label is singular, but the file is plural. All the
rest of the terms in the glossary prefer singular for the glossary
reference. Harmonizing the label entry is helpful for consistency.

Update all in tree tooltip references at the same time.

_data/glossary/label.yaml

@@ -0,0 +1,10 @@
+id: label
+name: Label
+full-link: /docs/concepts/overview/working-with-objects/labels
+tags:
+- fundamental
+short-description: >
+  Tags objects with identifying attributes that are meaningful and relevant to users.
+long-description: >
+  Labels are key/value pairs that are attached to objects such as {% glossary_tooltip text="Pods" term_id="pod" %}.
+  They are used to organize and to select subsets of objects.