Remove temporary dashboard image override before 14.0 ships

## Background

PR #20037 advances `release/14.0` to product version `14.0.0-preview.1`. Because `mcr.microsoft.com/dotnet/nightly/aspire-dashboard:14.0` was not yet published, it also adds a temporary `AspireDashboardImageTag=13.6` override so Docker Compose and Kubernetes publishing continue to use the available dashboard image.

Keep that workaround for now. This issue tracks undoing the temporary dashboard override for the 14.0 release once the matching image is available, without undoing the product version bump.

## Removal condition

**The `mcr.microsoft.com/dotnet/nightly/aspire-dashboard:14.0` tag must be published and pullable on the platforms used by the deployment tests before removing the workaround.**

## Work to complete

- Remove the temporary `AspireDashboardImageTag` property from `eng/Versions.props`.
- Remove the corresponding assembly metadata entries in `Aspire.Hosting.Docker.csproj` and `Aspire.Hosting.Kubernetes.csproj`, and the temporary override handling in `src/Shared/DashboardImage.cs`.
- Restore the existing product-version-based tag resolution introduced by #19249, so 14.0 publisher assemblies select the `14.0` dashboard image. Do not change the product version or revert to `latest`.
- Remove or adjust override-specific tests, retaining coverage of informational-version parsing, assembly-version fallback, and the running publisher assembly.
- Regenerate and review the affected Docker, Kubernetes, and Yarp publishing snapshots to expect the `14.0` image tag.
- Confirm the publisher tests and Docker Compose/Kubernetes deployment E2E checks pass with the published `14.0` image.

## Scope

This is cleanup of the temporary workaround in #20037, targeted for milestone **14.0**. It does not redesign the existing image-selection policy or address the unrelated Deno dependency-age failure.

First verify that mcr.microsoft.com/dotnet/nightly/aspire-dashboard:14.0 is published and pullable on deployment-test platforms. Then inspect eng/Versions.props, Aspire.Hosting.Docker.csproj, Aspire.Hosting.Kubernetes.csproj, and src/Shared/DashboardImage.cs, along with override-specific publisher tests and Docker, Kubernetes, and Yarp snapshots. Done means the temporary override is removed, 14.0 resolution and required parsing coverage remain, snapshots use the 14.0 tag, and publisher plus deployment E2E checks pass.

Written by the indexing model from the issue text.