Stop producing the Windows x86 SDK distribution in .NET 12

### Is your feature request related to a problem? Please describe.

We currently produce and publish a native Windows x86 (32-bit) SDK even though modern Windows installations and development environments are 64-bit. Most developers who need to build Windows x86 applications can use the x64 SDK and target or publish for x86 without running an x86 SDK.

The available usage signals indicate limited use:

- Microsoft Update (MU) data indicates Windows x86 is approximately 0.2% of SDK updates.
- .NET CLI telemetry reports Windows x86 at approximately 1.7% of Windows SDK usage.

Continuing to build, test, sign, publish, and service this distribution has a cost disproportionate to its observed usage. Before making the change, we want to identify customers or workflows that have a hard dependency on running the SDK itself as an x86 process.

### Describe the solution you'd like

Starting with .NET 12, stop producing and publishing the Windows x86 SDK distribution. This includes:

- Stop producing the Windows x86 SDK ZIP, TAR, and EXE installer artifacts.
- Remove the Windows x86 SDK from the download table.
- Stop including the Windows x86 SDK in `release.json` metadata.
- Remove the Windows x86 SDK from the .NET download page.

This proposal is limited to the native Windows x86 SDK distribution. It does not propose removing the ability to target or publish Windows x86 applications from a supported SDK architecture, and it does not propose removing Windows x86 runtime support.

Artifacts for currently supported .NET releases would continue to follow their existing servicing lifecycle. The change would apply to .NET 12 and later.

### Feedback requested

If you depend on the Windows x86 SDK, please describe:

- The scenario that requires the SDK process itself to run as x86.
- Why the x64 or Arm64 SDK cannot be used to build or publish the application.
- Whether the dependency is on the EXE installer, archive artifacts, release metadata, or another distribution surface.
- What migration time or tooling would be required.

### Additional context

This supersedes #52628, which raised the same question without a specific release target or the current usage data and artifact scope.

Related historical issue: dotnet/installer#8624.

Start by reviewing the proposed .NET 12 scope, the release.json metadata, download table, download page, and ZIP, TAR, and EXE artifact surfaces named in the issue. Check the superseded #52628 and related dotnet/installer#8624 for prior context, then identify affected release workflows and customer dependencies. Done means the Windows x86 SDK is no longer produced or published for .NET 12 and later while x86 targeting, publishing, and runtime support remain unchanged.

Written by the indexing model from the issue text.