Remove code that uses the ConnectionStrings__<name> environment variable

[alistairmatthews]

Although in the Dashboard there is often an environment variable such as ConnectionStrings__postgresdb listed, this shouldn't be used in code for two reasons:

• The double-underscore returns null and should be replaced with a colon.
• We should always encourage the Aspire method instead: var connectionUri = builder.Configuration.GetConnectionString("postgresdb");

Search for all instances of builder.Configuration.GetValue<string>("ConnectionStrings__<name>");
and replace them with: builder.Configuration.GetConnectionString("postgresdb");

[afscrome]

This does assume that you're using the connection string in a .Net App. If you're consuming the connection string in a non .Net app, you'll need to use the raw environment variable, so I think the docs should still document the raw environment variable.