Switch from main/master to latest/latest on CI

The main/master pair was added to the CI to ensure the integration
keeps working on to-be-released versions of OTP and Elixir. Currently,
most likely because of a change upstream, the CI is failing. This
patch replaces the main/master pair with the latest released version
of each.

[skip changeset]

.github/workflows/ci.yaml

@@ -129,8 +129,8 @@ jobs:
           - 26.x
           - 25.x
         include:
-          - elixir: main
-            otp: master
+          - elixir: latest
+            otp: latest
           - elixir: 1.16.x
             otp: 26.x
           - elixir: 1.15.x