[release-1.4] cleanup: Add missing BackendTLSPolicy GA reference and switch SAN validation feature to standard (#4194)

* cleanup: add BackendTLSPolicy as GA in README.md

Signed-off-by: Norwin Schnyder <norwin.schnyder+github@gmail.com>

* cleanup: SAN validation in BTLSP is no longer experimental

Signed-off-by: Norwin Schnyder <norwin.schnyder+github@gmail.com>

---------

Signed-off-by: Norwin Schnyder <norwin.schnyder+github@gmail.com>
Co-authored-by: Norwin Schnyder <norwin.schnyder+github@gmail.com>

Author: k8s-infra-cherrypick-robot

README.md

@@ -14,6 +14,7 @@ This version of the API is has GA level support for the following resources:
 - `v1.Gateway`
 - `v1.HTTPRoute`
 - `v1.GRPCRoute`
+- `v1.BackendTLSPolicy`
 
 For all the other APIs and their support levels please consult [the spec][spec].
 

pkg/features/backendtlspolicy.go

@@ -40,7 +40,7 @@ var BackendTLSPolicyFeature = Feature{
 // SubjectAltName Validation feature.
 var BackendTLSPolicySanValidationFeature = Feature{
 	Name:    SupportBackendTLSPolicySANValidation,
-	Channel: FeatureChannelExperimental,
+	Channel: FeatureChannelStandard,
 }
 
 // BackendTLSPolicyCoreFeatures includes all the supported features for the