cleanup(docs): update docs to reflect BackendTLSPolicy GA (#4204)

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

Author: snorwin

site-src/api-types/backendtlspolicy.md

@@ -1,10 +1,10 @@
 # BackendTLSPolicy
 
-??? example "Experimental Channel since v1.0.0"
+??? success "Standard Channel since v1.4.0"
 
-    The `BackendTLSPolicy` resource is Alpha and has been part of
-    the Experimental Channel since `v1.0.0`. For more information on release
-    channels, refer to our [versioning guide](../concepts/versioning.md).
+    The `BackendTLSPolicy` resource is GA and has been part of the Standard
+    Channel since `v1.4.0`. For more information on release channels, refer
+    to our [versioning guide](../concepts/versioning.md).
 
 [BackendTLSPolicy][backendtlspolicy] is a Gateway API type for specifying the TLS configuration
 of the connection from the Gateway to a backend pod/s via the Service API object.

site-src/guides/tls.md

@@ -116,8 +116,8 @@ would be invalid.
 
 ## Upstream TLS
 
-Upstream TLS settings are configured using the experimental `BackendTLSPolicy`
-attached to a `Service` via a target reference.
+Upstream TLS settings are configured using the `BackendTLSPolicy` attached to a
+`Service` via a target reference.
 
 This resource can be used to describe the SNI the Gateway should use to connect to the
 backend and how the certificate served by the backend Pod(s) should be verified.