Consider replacing AddRouting with AddRoutingCore in AddConnections

[eerhardt]

In #56460 (comment) we are making SignalR server compatible with native AOT. Investigating the extra 2.2 MB of app size in a SignalR app over the dotnet new webapiaot template, the largest difference that can be trimmed is from cutting System.Text.RegularExpressions out of the app. The reason it gets pulled in is because this calls AddRouting and not AddRoutingCore.

aspnetcore/src/SignalR/common/Http.Connections/src/ConnectionsDependencyInjectionExtensions.cs

Lines 21 to 23 in 0da8ea7

	public static IServiceCollection AddConnections(this IServiceCollection services)
	{
	services.AddRouting();

In the past investigations, Regex adds roughly 750 KB of app size in native AOT (see #46142 and #46227). We added AddRoutingCore for this specific reason.

cc @BrennanConroy