Add currently supported elixir versions to the test matrix

* Includes the highest OTP version mentioned in https://hexdocs.pm/elixir/compatibility-and-deprecations.html#between-elixir-and-erlang-otp

.github/workflows/test.yml

@@ -8,8 +8,17 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        otp: ['25.3', '26.2']
-        elixir: ['1.16.0']
+        include:
+          - elixir: 1.17.x
+            otp: 27
+          - elixir: 1.16.x
+            otp: 26
+          - elixir: 1.15.x
+            otp: 26
+          - elixir: 1.14.x
+            otp: 26
+          - elixir: 1.13.x
+            otp: 25
 
     services:
       postgres: