[vsphere-ci] Specify VM hardware version to 15 in Packer build script

Developer story

As a WMCO developer, I want to set the VM hardware version to 15 in the Packer build script, so that the resulting template uses a supported compatibility level. See WINC-725

Description

Update the Building the vSphere Windows VM Golden Image programmatically guide to ensure the resulting template uses a hardware version supported by WMCO.

Engineering Details

Follow Packer documentation to include the`vm_version` setting in the build script  to set the VM hardware version.

Avoid defaulting to the most current VM hardware version supported by vCenter.

See VMWare article 1003746 for the full list of supported VM hardware versions.

Acceptance Criteria

• Hardware version is set 15
• The resulting golden image shows the configured hardware version
• WMCO' vSphere golden image documentation points to VMware vSphere infrastructure requirements
• WMCO' user-facing documentation for vSphere also points to VMware vSphere infrastructure requirements