[Fixes] VMSS enabledAcceleratedNetworking fix

[lsnoddy]

Description

Fixed bug in enabling AcceleratedNetworking on VMSS

• Changed code to check for "enabledAcceleratedNetworking" property existence on the indexed array element instead of the outer array
• Fixed the same issue for the "networkSecurityGroup" property
• Changed "enabledAcceleratedNetworking" default status from null to true to match default behavior in the portal
• Updated windows and linux paramter files to a new VM sku compatible with Accelerated Networking; used the same sku as VMs (Standard_B12ms)
• Updated the readme

Pipeline references

For module/pipeline changes, please create and attach the status badge of your successful run.

Pipeline

Type of Change

Please delete options that are not relevant.

• Bugfix (non-breaking change which fixes an issue)

Checklist

• I'm sure there are no other open Pull Requests for the same update/change
• My corresponding pipelines / checks run clean and green without any errors or warnings
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (readme)
• I did format my code