Add otp 28 to CI

Author: cschiewek

.github/workflows/elixir.yml

@@ -14,16 +14,23 @@ jobs:
     strategy:
       matrix:
         elixir:
-          - '1.15'
-          - '1.16'
-          - '1.17'
-          - '1.18'
+          - "1.15"
+          - "1.16"
+          - "1.17"
+          - "1.18"
         otp:
-          - '25'
-          - '26'
-          - '27'
+          - "25"
+          - "26"
+          - "27"
+          - "28"
         # see https://hexdocs.pm/elixir/compatibility-and-deprecations.html#between-elixir-and-erlang-otp
         exclude:
+          - elixir: 1.17
+            otp: 28
+          - elixir: 1.16
+            otp: 28
+          - elixir: 1.15
+            otp: 28
           - elixir: 1.16
             otp: 27
           - elixir: 1.15