Promote Gateway for Mesh (GAMMA) to Standard (#2873)

This PR raises Gateway API for Mesh to "Standard" channel.

What is included:
* GEP-1426 (Routes binding to service) and GEP-1686 (conformance tests
  for mesh)
* Updated various docs to declare stable.

Followup needed:
* Blog post announcement

What is *not* included:
* Changes to any tests; we already have existing conformance for mesh,
  and the `feature` is already called `MeshCoreFeatures` so I don't
think we need to change anything there. In the future, we may add
experimental features to Mesh, at which point we can make a new set of
features like `MeshExperimentalFeatures` or similar.
* Changes to any Go/CRD APIs beyond docs. The Gateway for Mesh work did not
  introduce any API changes. Instead it just introduced new ways to use
the existing APIs with corresponding documentation changes to remove the
'Experimental' signifier.

apis/v1/shared_types.go

@@ -25,7 +25,7 @@ import (
 // with "Core" support:
 //
 // * Gateway (Gateway conformance profile)
-// * Service (Mesh conformance profile, experimental, ClusterIP Services only)
+// * Service (Mesh conformance profile, ClusterIP Services only)
 //
 // This API may be extended in the future to support additional kinds of parent
 // resources.
@@ -49,7 +49,7 @@ type ParentReference struct {
 	// There are two kinds of parent resources with "Core" support:
 	//
 	// * Gateway (Gateway conformance profile)
-	// * Service (Mesh conformance profile, experimental, ClusterIP Services only)
+	// * Service (Mesh conformance profile, ClusterIP Services only)
 	//
 	// Support for other resources is Implementation-Specific.
 	//
@@ -96,9 +96,7 @@ type ParentReference struct {
 	// both specified values.
 	// * Service: Port Name. When both Port (experimental) and SectionName
 	// are specified, the name and port of the selected listener must match
-	// both specified values. Note that attaching Routes to Services as Parents
-	// is part of experimental Mesh support and is not supported for any other
-	// purpose.
+	// both specified values.
 	//
 	// Implementations MAY choose to support attaching Routes to other resources.
 	// If that is the case, they MUST clearly document how SectionName is
@@ -170,9 +168,8 @@ type CommonRouteSpec struct {
 	// There are two kinds of parent resources with "Core" support:
 	//
 	// * Gateway (Gateway conformance profile)
-	// <gateway:experimental:description>
-	// * Service (Mesh conformance profile, experimental, ClusterIP Services only)
-	// </gateway:experimental:description>
+	// * Service (Mesh conformance profile, ClusterIP Services only)
+	//
 	// This API may be extended in the future to support additional kinds of parent
 	// resources.
 	//