Remove deprecated ubuntu-18 runner from CI

Github deprecated ubuntu 18 runners, see actions/runner-images#6002
We have to run on ubuntu 20. However the setup-beam action only supports
OTP 20+ on ubuntu 20, so we update that too

Author: michalmuskala

.github/workflows/ci.yml

@@ -16,17 +16,17 @@ jobs:
       matrix:
         include:
           - elixir: 1.4.x
-            otp: 18
-            os: ubuntu-18.04
+            otp: 20
+            os: ubuntu-20.04
           - elixir: 1.5.x
-            otp: 18
-            os: ubuntu-18.04
+            otp: 20
+            os: ubuntu-20.04
           - elixir: 1.6.x
-            otp: 19
-            os: ubuntu-18.04
+            otp: 20
+            os: ubuntu-20.04
           - elixir: 1.7.x
-            otp: 19
-            os: ubuntu-18.04
+            otp: 20
+            os: ubuntu-20.04
           - elixir: 1.8.x
             otp: 20
             os: ubuntu-20.04