Add RIDs for RHEL 7.3 and 7.4 (dotnet/corefx#15006)

Commit migrated from https://github.com/dotnet/corefx/commit/68c03688188cb2fd8adbfd12cadc5c6f30f99b43

src/libraries/pkg/Microsoft.NETCore.Platforms/runtime.json

@@ -230,6 +230,20 @@
             "#import": [ "rhel.7.2", "rhel.7.1-x64" ]
         },
 
+        "rhel.7.3": {
+            "#import": [ "rhel.7.2" ]
+        },
+        "rhel.7.3-x64": {
+            "#import": [ "rhel.7.3", "rhel.7.2-x64" ]
+        },
+
+        "rhel.7.4": {
+            "#import": [ "rhel.7.3" ]
+        },
+        "rhel.7.4-x64": {
+            "#import": [ "rhel.7.4", "rhel.7.3-x64" ]
+        },
+
         "ol": {
             "#import": [ "rhel" ]
         },