Remove `MetadataHelpers.GenericParameterAttributesAllowByRefLike` in favor of `GenericParameterAttributes.AllowByRefLike`

[AlekseyTs]

        // PROTOTYPE(RefStructInterfaces): Remove this constant and refer to GenericParameterAttributes.AllowByRefLike directly once the new enum member becomes available.
        //                                 See // https://github.com/dotnet/runtime/issues/68002#issuecomment-1942166436 for more details.
        public const System.Reflection.GenericParameterAttributes GenericParameterAttributesAllowByRefLike = (System.Reflection.GenericParameterAttributes)0x0020;

[jaredpar]

This can't be done until we move to SCI version 9.0. That will happen likely after 17.12 but by the time we are in 17.12 we'll have a more appropriate milestone to put this in.