renovate: limit pdfjs-dist to <3

They bumped the browser requirements quite a bit:
https://github.com/mozilla/pdf.js/commit/b7f987a18549e67333c6a65

Let's wait a bit to not drop safari <14 support

.renovaterc.json

@@ -5,5 +5,11 @@
     "composer": {
         "enabled": false
     },
-    "cloneSubmodules": true
+    "cloneSubmodules": true,
+    "packageRules": [
+        {
+          "matchPackageNames": ["pdfjs-dist"],
+          "allowedVersions": "<3"
+        }
+    ]
 }