Drop support for GraphQL 15

Author: PowerKiKi

.changeset/shaggy-rabbits-compare.md

@@ -0,0 +1,5 @@
+---
+'apollo-angular': major
+---
+
+Drop support for GraphQL 15

.github/workflows/main.yml

@@ -113,7 +113,7 @@ jobs:
     strategy:
       matrix:
         angular_version: [18, 19, 20]
-        graphql_version: [15, 16]
+        graphql_version: [16]
         node_version: [18, 20, 22, 24]
         exclude:
           - angular_version: 18

packages/apollo-angular/package.json

@@ -38,7 +38,7 @@
   "peerDependencies": {
     "@angular/core": "^18.0.0 || ^19.0.0 || ^20.0.0",
     "@apollo/client": "^3.13.1",
-    "graphql": "^15.0.0 || ^16.0.0",
+    "graphql": "^16.0.0",
     "rxjs": "^6.0.0 || ^7.0.0"
   },
   "dependencies": {