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/selector.yaml

@@ -9,5 +9,5 @@ short-description: >
   Allows users to filter a list of resources based on labels.
 long-description: >
   Selectors are applied when querying lists of resources to filter
-  them by {% glossary_tooltip text="Labels" term_id="labels"
+  them by {% glossary_tooltip text="Labels" term_id="label"
   %}.

docs/user-journeys/users/application-developer/advanced.md

@@ -34,7 +34,7 @@ As you may know, it's an antipattern to migrate an entire app (e.g. containerize
 
 #### Pod configuration
 
-Usually, you use {% glossary_tooltip text="labels" term_id="labels" %} and {% glossary_tooltip text="annotations" term_id="annotation" %} to attach metadata to your resources. To inject data into your resources, you'd likely create {% glossary_tooltip text="ConfigMaps" term_id="configmap" %} (for nonconfidential data) or {% glossary_tooltip text="Secrets" term_id="secret" %} (for confidential data).
+Usually, you use {% glossary_tooltip text="labels" term_id="label" %} and {% glossary_tooltip text="annotations" term_id="annotation" %} to attach metadata to your resources. To inject data into your resources, you'd likely create {% glossary_tooltip text="ConfigMaps" term_id="configmap" %} (for nonconfidential data) or {% glossary_tooltip text="Secrets" term_id="secret" %} (for confidential data).
 
 Below are some other, lesser-known ways of configuring your resources' Pods:
 

docs/user-journeys/users/application-developer/foundational.md

@@ -71,7 +71,7 @@ Through these deployment tasks, you'll gain familiarity with the following:
 * Common workload objects
   * **{% glossary_tooltip text="Deployment" term_id="deployment" %}** - The most common way of running *X* copies (Pods) of your application. Supports rolling updates to your container images.
 
-  * **{% glossary_tooltip text="Service" term_id="deployment" %}** - By itself, a Deployment can't receive traffic. Setting up a Service is one of the simplest ways to configure a Deployment to receive and loadbalance requests. Depending on the `type` of Service used, these requests can come from external client apps or be limited to apps within the same cluster. A Service is tied to a specific Deployment using {% glossary_tooltip text="label" term_id="labels" %} selection.
+  * **{% glossary_tooltip text="Service" term_id="deployment" %}** - By itself, a Deployment can't receive traffic. Setting up a Service is one of the simplest ways to configure a Deployment to receive and loadbalance requests. Depending on the `type` of Service used, these requests can come from external client apps or be limited to apps within the same cluster. A Service is tied to a specific Deployment using {% glossary_tooltip text="label" term_id="label" %} selection.
 
 The subsequent topics are also useful to know for basic application deployment.
 
@@ -79,7 +79,7 @@ The subsequent topics are also useful to know for basic application deployment.
 
 You can also specify custom information about your Kubernetes API objects by attaching key/value fields. Kubernetes provides two ways of doing this:
 
-* **{% glossary_tooltip text="Labels" term_id="labels" %}** - Identifying metadata that you can use to sort and select sets of API objects. Labels have many applications, including the following:
+* **{% glossary_tooltip text="Labels" term_id="label" %}** - Identifying metadata that you can use to sort and select sets of API objects. Labels have many applications, including the following:
 
   * *To keep the right number of replicas (Pods) running in a Deployment.* The specified label (`app: nginx` in the [stateless app example](/docs/tasks/run-application/run-stateless-application-deployment/#creating-and-exploring-an-nginx-deployment){:target="_blank"}) is used to stamp the Deployment's newly created Pods (as the value of the `spec.template.labels` configuration field), and to query which Pods it already manages (as the value of `spec.selector.matchLabels`).