Run CI on ubuntu-20.04 (elixirmoney#200)

This is because the ubuntu-latest is resolved ubuntu-22.04,
which is not compatible with otp 21.3 which this project supports.

This also updates actions/checkout from v2 to v3,
consequently resolving deprecation warning that prompts using Nodejs v16.

.github/workflows/ci.yml

@@ -2,7 +2,7 @@ on: push
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     name: OTP ${{matrix.otp}} / Elixir ${{matrix.elixir}}
     strategy:
       matrix:
@@ -14,10 +14,10 @@ jobs:
 
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: erlef/setup-beam@v1
         with:
           otp-version: ${{matrix.otp}}
           elixir-version: ${{matrix.elixir}}
       - run: mix deps.get
-      - run: mix test
+      - run: mix test