docs(graphql): SchemaDirectiveVisitor requires @graphql-tools/utils@^7

Fixes #2021

Author: sgarner

content/graphql/directives.md

@@ -10,7 +10,7 @@ A directive is an identifier preceded by a `@` character, optionally followed by
 
 #### Custom directives
 
-To create a custom schema directive, declare a class which extends the `SchemaDirectiveVisitor` class exported from the `apollo-server` package.
+To create a custom schema directive, declare a class which extends the `SchemaDirectiveVisitor` class exported from the `@graphql-tools/utils` package.
 
 ```typescript
 import { SchemaDirectiveVisitor } from '@graphql-tools/utils';
@@ -32,7 +32,8 @@ export class UpperCaseDirective extends SchemaDirectiveVisitor {
 
 > info **Hint** Note that directives cannot be decorated with the `@Injectable()` decorator. Thus, they are not able to inject dependencies.
 > 
-> warning **Warning** `SchemaDirectiveVisitor` is exported from the `@graphql-tools/utils` package so make sure to install it in your project.
+> warning **Warning** `SchemaDirectiveVisitor` is exported from the `@graphql-tools/utils` package. Note that the 8.x release of this package removes this export
+> and uses a different and incompatible approach to directives, so make sure to install `@graphql-tools/utils@^7` in your project.
 
 Now, register the `UpperCaseDirective` in the `GraphQLModule.forRoot()` method: