test(opentelemetry-node): downgrade to undici@6 for testing

undici@7 dropped support for Node.js v18 (which this package still
supports) and finally *broke* v18 support in undici@7.13.0

Refs: #949 (comment)

Author: trentm

.github/dependabot.yml

@@ -16,6 +16,10 @@ updates:
       # Do not yet support openai@5.
       - dependency-name: "openai"
         versions: [ ">=5" ]
+      # Packages whose major versions have dropped support for Node.js versions
+      # that this package needs.
+      - dependency-name: "undici" # undici@7 dropped 18
+        versions: [ ">=7" ]
     groups:
       opentelemetry:
         patterns:

packages/opentelemetry-node/package-lock.json

@@ -169,7 +169,7 @@
     "semver": "^7.6.3",
     "tape": "^5.9.0",
     "tedious": "^18.6.1",
-    "undici": "^7.1.0",
+    "undici": "^6.21.3",
     "winston": "^3.13.1"
   }
 }