Remove mentions of mixed

Author: JoviDeCroock

website/pages/api-v16/error.mdx

@@ -56,7 +56,7 @@ class GraphQLError extends Error {
     source?: Source,
     positions?: number[],
     originalError?: Error,
-    extensions?: { [key: string]: mixed },
+    extensions?: Record<string, unknown>,
   );
 }
 ```

website/pages/api-v16/execution.mdx

@@ -71,9 +71,9 @@ to be removed in v17.
 export function execute(
   schema: GraphQLSchema,
   documentAST: Document,
-  rootValue?: mixed,
-  contextValue?: mixed,
-  variableValues?: { [key: string]: mixed },
+  rootValue?: unknown,
+  contextValue?: unknown,
+  variableValues?: Record<string, unknown>,
   operationName?: string,
 ): MaybePromise<ExecutionResult>;
 ```
@@ -138,9 +138,9 @@ to be removed in v17.
 export function executeSync(
   schema: GraphQLSchema,
   documentAST: Document,
-  rootValue?: mixed,
-  contextValue?: mixed,
-  variableValues?: { [key: string]: mixed },
+  rootValue?: unknown,
+  contextValue?: unknown,
+  variableValues?: Record<string, unknown>,
   operationName?: string,
 ): ExecutionResult;
 ```