Migrate GitHub Actions to erlef/setup-beam

See https://github.com/erlef/setup-beam/issues/20

We also bump to latest OTP 25.

.github/workflows/ci.yml

@@ -25,22 +25,25 @@ jobs:
             no-warnings-as-errors: true
           - pair:
               elixir: 1.12.3
-              otp: 24.3
+              otp: 24.3.4
 # Although Elixir 1.12 supports this feature, we're expecting errors since the code to suppress
 # undefined-function-warnings on RDF.Graph.build blocks relies on Elixir 1.13 and is disabled
 # partially on older versions.
             no-warnings-as-errors: true
           - pair:
-              elixir: 1.13.3
+              elixir: 1.13.4
               otp: 23.3
           - pair:
-              elixir: 1.13.3
+              elixir: 1.13.4
               otp: 24.3
+          - pair:
+              elixir: 1.13.4
+              otp: 25.0
             lint: lint
     steps:
       - uses: actions/checkout@v2
 
-      - uses: erlef/setup-elixir@v1
+      - uses: erlef/setup-beam@v1
         with:
           otp-version: ${{matrix.pair.otp}}
           elixir-version: ${{matrix.pair.elixir}}