chore: disable dependabot updates of the test app used for the experi…

…mental Next.js instrumentation (#4415)

The next.js instrumentation is experimental and support is locked to an
old/deprecated version of Next.js and is unlikely to move forward from
there. There is no value in dependabot updates for this test app.

.github/dependabot.yml

@@ -55,14 +55,6 @@ updates:
     reviewers:
       - "elastic/apm-agent-node-js"
 
-  - package-ecosystem: "npm"
-    directory: "/test/instrumentation/modules/next/a-nextjs-app"
-    schedule:
-      interval: "weekly"
-    open-pull-requests-limit: 5
-    reviewers:
-      - "elastic/apm-agent-node-js"
-
   - package-ecosystem: "npm"
     directory: "/test/opentelemetry-bridge"
     schedule: