Bump OTP to 26 for mixed version tests

Mixed version tests use the same erlang for all nodes, so the erlang version must be supported by both new and old version.

Author: ansd

.github/workflows/test-mixed-versions.yaml

@@ -127,7 +127,7 @@ jobs:
       fail-fast: false
       matrix:
         otp_version_id:
-        - "25_3"
+        - "26"
     timeout-minutes: 120
     steps:
     - name: CHECKOUT REPOSITORY
@@ -171,7 +171,7 @@ jobs:
     strategy:
       matrix:
         include:
-        - erlang_version: "25.3"
+        - erlang_version: "26.0"
           elixir_version: 1.14.5
     timeout-minutes: 60
     steps: