Use actions/cache@v3 on CI

.github/workflows/main.yml

@@ -40,7 +40,7 @@ jobs:
       # Don't cache PLTs based on mix.lock hash, as Dialyzer can incrementally update even old ones
       # Cache key based on Elixir & Erlang version (also useful when running in matrix)
       - name: Cache Dialyzer's PLT
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         id: cache-plt
         with:
           path: priv/plts