Update runners to ubuntu-22.04 (docs.yml) (#1056)

Continuation of https://github.com/JetBrains/skiko/pull/1055

This runs only in master

.github/workflows/docs.yml

@@ -12,7 +12,7 @@ on:
 
 jobs:
   dokka:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v3
         name: 'Check out code'