Use different version of rebar3 for OTP 21 in github actions.

.github/workflows/ci.yml

@@ -9,16 +9,20 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
-    name: Tests on ${{matrix.otp}}
+    name: Tests on OTP ${{matrix.otp}} with rebar v${{matrix.rebar3}}
     strategy:
       matrix:
-        otp: ['24.1.2', '23.3.4.7', '22.3.4.22', '21.3.8.24']
+        otp: ['24.1.2', '23.3.4.7', '22.3.4.22']
+        rebar3: ['3.17.0']
+        include:
+          - otp: '21.3.8.24'
+            rebar3: '3.15.1'
     steps:
       - uses: actions/checkout@v2
       - uses: erlef/setup-beam@v1
         with:
           otp-version: ${{matrix.otp}}
-          rebar3-version: 3.15.1
+          rebar3-version: ${{matrix.rebar3}}
       - name: Common Test tests
         run: rebar3 ct --sname ct
       - name: Dialyzer