Portal Accelerator: Remove regional VPN Gateways option

[Springstone]

This pull request updates the Azure Landing Zones (ALZ) portal accelerator to remove support for regional VPN gateways, which are being deprecated. The changes ensure that all supported regions now deploy zone redundant VPN gateways by default, simplifying deployment options and aligning with current best practices.

Tooling and Portal Accelerator Updates

• Removed the option to select regional VPN gateways from the ALZ portal accelerator UI, so users can only deploy zone redundant VPN gateways going forward. [1] [2]
• Deleted configuration parameters and UI elements related to regional VPN gateway SKU selection for both primary and secondary regions (gwRegionalSku, gwRegionalSkuSecondary). [1] [2]
• Updated visibility logic for VPN gateway SKU selection so it only applies to zone redundant gateways. [1] [2]

Default Behavior Changes

• Set the default value for gateway deployment type to "Zone" in both ARM templates and portal configuration, ensuring zone redundant gateways are deployed by default. [1] [2]
• Hardcoded "gwRegionalOrAz" and "gwRegionalOrAzSecondary" to "Zone" in deployment parameters, removing dynamic selection logic. [1] [2]

Documentation

• Updated docs/wiki/Whats-new.md to document the removal of regional VPN gateway support and the new default behavior.