Update GraphQL Playground to v1.7.32.

This fixes a small, rogue, often invisible curly brace that was appearing on
the top-right side of GraphQL Playground's interface.  See referenced issue
for details.

Ref: apollographql/graphql-playground#21
Cc: @tizmagik

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

@@ -10,7 +10,7 @@ export {
 // This specifies the version of `graphql-playground-react` that will be served
 // from `graphql-playground-html`.  It's passed to ``graphql-playground-html`'s
 // renderPlaygroundPage` via the integration packages' playground configuration.
-const playgroundVersion = '1.7.31';
+const playgroundVersion = '1.7.32';
 
 // https://stackoverflow.com/a/51365037
 type RecursivePartial<T> = {