SFI: Remove passwords from WebSdk/README.md

The [WebSdk/README.md](https://github.com/dotnet/sdk/blob/main/src/WebSdk/README.md) contains `password` 31 times. Per [this comment](https://github.com/dotnet/sdk/pull/44761#issuecomment-2465962140) by @baronfel:

> I'd strongly consider changing the doc to some mechanism that does not ingest secrets of any kind into MSBuild via Properties - once something is an MSBuild property it is visible to all Task and MSBuild logic that runs in that same build, so unless you are strictly auditing the build logic you use (including build logic from NuGet packages, etc) you're exposing yourself.