Graduate Helm support

There was a disclaimer in the documentation that the Helm installation
method was still considered "Alpha".

We now consider this installation method stable, and we remove the
disclaimer for the Antrea v2.0 release.

For antrea-io#4832

Signed-off-by: Antonin Bas <antonin.bas@broadcom.com>

docs/helm.md

@@ -29,8 +29,6 @@ We provide the following Helm charts:
 Note that these charts are the same charts that we use to generate the YAML
 manifests for the `kubectl apply` installation method.
 
-Helm installation is currently considered Alpha.
-
 ## Prerequisites
 
 * Ensure that the necessary