Remove the feature gate part from immutable secrets/configmaps

The feature is in `beta` since Kubernetes v1.19.0 so it is enabled per
default. This means that we can omit the hint to enable the feature
gate manually.

Signed-off-by: Sascha Grunert <[email protected]>

Author: saschagrunert

content/en/docs/concepts/configuration/configmap.md

@@ -224,9 +224,10 @@ data has the following advantages:
 - improves performance of your cluster by significantly reducing load on kube-apiserver, by
 closing watches for config maps marked as immutable.
 
-To use this feature, enable the `ImmutableEphemeralVolumes`
-[feature gate](/docs/reference/command-line-tools-reference/feature-gates/) and set
-your Secret or ConfigMap `immutable` field to `true`. For example:
+This feature is controlled by the `ImmutableEphemeralVolumes` [feature
+gate](/docs/reference/command-line-tools-reference/feature-gates/),
+which is enabled by default since v1.19. You can create an immutable
+ConfigMap by setting the `immutable` field to `true`. For example,
 ```yaml
 apiVersion: v1
 kind: ConfigMap

content/en/docs/concepts/configuration/secret.md

@@ -728,9 +728,10 @@ data has the following advantages:
 - improves performance of your cluster by significantly reducing load on kube-apiserver, by
 closing watches for secrets marked as immutable.
 
-To use this feature, enable the `ImmutableEphemeralVolumes`
-[feature gate](/docs/reference/command-line-tools-reference/feature-gates/) and set
-your Secret or ConfigMap `immutable` field to `true`. For example:
+This feature is controlled by the `ImmutableEphemeralVolumes` [feature
+gate](/docs/reference/command-line-tools-reference/feature-gates/),
+which is enabled by default since v1.19. You can create an immutable
+Secret by setting the `immutable` field to `true`. For example,
 ```yaml
 apiVersion: v1
 kind: Secret