Pin CI to Elixir 1.20.2 and add the OTP 29 matrix entry

Author: kipcole9

.github/workflows/ci.yml

@@ -43,10 +43,12 @@ jobs:
             otp: "28"
 
           # Elixir 1.20 — supports OTP 27–28
-          - elixir: "1.20.0-rc.4-otp-27"
+          - elixir: "1.20.2-otp-27"
             otp: "27"
-          - elixir: "1.20.0-rc.4-otp-28"
+          - elixir: "1.20.2-otp-28"
             otp: "28"
+          - elixir: "1.20.2-otp-29"
+            otp: "29"
             lint: true
 
     steps: