Update install script to avoid relying on dotnet/aspire → microsoft/aspire repo redirect

### Problem
The current install script (https://aspire.dev/install.ps1) still points to the old `dotnet/aspire` repo path. This works today because GitHub provides a temporary redirect when a repository is transferred between organizations, but that redirect is only guaranteed for a limited time (usually 6 months, but maybe Microsoft treats its own things differently?).

If the redirect expires and the script isn’t updated to use the new `microsoft/aspire` location directly, installation will break for all users relying on the script.

### Recommendation
- Update all install-related URLs and script logic to point directly to `microsoft/aspire` instead of `dotnet/aspire`.
- Verify all docs and references (including in official docs and the CLI installer landing page) are also updated.

### Context
- [Install script redirection example](https://aspire.dev/install.ps1 → https://raw.githubusercontent.com/dotnet/aspire/e838b86b5dcbf2f4cd136dff37826ad5e84f6b6c/eng/scripts/get-aspire-cli.ps1)
- [GitHub’s repo-redirect policy](https://docs.github.com/en/repositories/creating-and-managing-repositories/transferring-a-repository#what-changes-after-a-repository-is-transferred)