Add beta label to MSI on install Elasticsearch page (#28126)

The main installation instructions page for the Windows MSI installer includes a header at the top to indicate that the installer is in beta, but the Installing Elasticsearch page does not. This commit adds the beta label to the MSI entry within the installation options.

docs/reference/setup/install.asciidoc

@@ -41,6 +41,8 @@ Elasticsearch website or from our RPM repository.
 
 `msi`::
 
+beta[]
++
 The `msi` package is suitable for installation on Windows 64-bit systems with at least
 .NET 4.5 framework installed, and is the easiest choice for getting started with
 Elasticsearch on Windows. MSIs may be downloaded from the Elasticsearch website.