chore(core): Downgrade nestjs/graphql due to issue

The latest version has a regression which affects the Elasticsearch plugin's customMappings field
resolver. See https://github.com/nestjs/graphql/issues/1364

packages/core/package.json

@@ -41,7 +41,7 @@
     "@graphql-tools/stitch": "^6.2.4",
     "@nestjs/common": "7.6.13",
     "@nestjs/core": "7.6.13",
-    "@nestjs/graphql": "7.9.11",
+    "@nestjs/graphql": "7.9.7",
     "@nestjs/platform-express": "7.6.13",
     "@nestjs/terminus": "7.1.0",
     "@nestjs/testing": "7.6.13",

yarn.lock

@@ -2980,10 +2980,10 @@
     tslib "2.1.0"
     uuid "8.3.2"
 
-"@nestjs/graphql@7.9.11":
-  version "7.9.11"
-  resolved "https://registry.npmjs.org/@nestjs/graphql/-/graphql-7.9.11.tgz#2bfaf9b2800cb7e71238e4d60376c3edf5f6e809"
-  integrity sha512-rht7xl02f8kSwAdXbOiR06d69QUCKSHjpbkMFIFuFUcDkvIRigaNoXs9MpFAECnvpi4w9o3F7KT4rn06ShwKMA==
+"@nestjs/graphql@7.9.7":
+  version "7.9.7"
+  resolved "https://registry.npmjs.org/@nestjs/graphql/-/graphql-7.9.7.tgz#9dd32ec58e72983348ae2633dc71101f86ba96af"
+  integrity sha512-opRLzQw6gwq4pbA8l6CewIH2Kr6hZy9VnHPu6jbrBc2WUrk+l0N9+uqZbWcnAxvthzfSxFTukgJKlepDv4yJAw==
   dependencies:
     "@graphql-tools/merge" "6.2.7"
     "@graphql-tools/schema" "6.2.4"