Add note to README.md that Windows containers are no longer published

[mloskot]

The documents in this repository contains number of references that easily make users try docker pull microsoft/mssql-server-windows-developer what, unfortunately, will not work due to Beta program for SQL Server on Windows container is suspended which is also referenced in #684 (comment) from @amvin87-zz

I think, it would safe time for users like myself if there was a big bold red notice at the top of the README.md

> [!IMPORTANT]
> Since the Beta program for SQL Server on Windows container has been suspended,
> Windows-based images are no longer published.