Revert "Revert "Add deprecation warning for engine.schemaTag""

This reverts commit 627594b.

Author: Adam Zionts

packages/apollo-server-core/src/ApolloServer.ts

@@ -98,6 +98,12 @@ function getEngineGraphVariant(engine: Config['engine']): string | undefined {
   if (engine === false) {
     return;
   } else if (typeof engine === 'object' && (engine.graphVariant || engine.schemaTag)) {
+    if (engine.graphVariant && engine.schemaTag) {
+      throw new Error('Cannot set both engine.graphVariant and engine.schemaTag. Please use engine.graphVariant.');
+    }
+    if (engine.schemaTag) {
+      console.warn('[Deprecation warning] Usage of engine.schemaTag is deprecated. Please use engine.graphVariant instead.');
+    }
     return engine.graphVariant || engine.schemaTag;
   } else {
     if (process.env.ENGINE_SCHEMA_TAG) {