Refactor gRPC content-type constraint to be inline

[JamesNK]

This method call brought in the .NET regex engine to apps published as trimmed or AOT:

        var routePattern = RoutePatternFactory.Parse(
            pattern,
            defaults: null,
            parameterPolicies: new RouteValueDictionary { ["contentType"] = GrpcUnimplementedConstraint.Instance });

This PR refactors routes to include unimplemented constraint inline.

PR also updates .NET 7 SDK to 7.0.201. This version is used by gRPC-Web tests docker image. See #2063 (comment)