Update christianliebel's socials (#3800)

Author: christianliebel

src/config/contributors.json

@@ -945,6 +945,8 @@
   "christianliebel": {
     "avatar_url": "6698344",
     "github": "christianliebel",
+    "linkedin": "christianliebel",
+    "mastodon": "https://mastodon.cloud/@christianliebel",
     "name": "Christian Liebel",
     "teams": {
       "2020": [