[CI] Updates otel GitHub Action and Buildkite yaml test branch for 8.18

Author: picandocodigo

.buildkite/pipeline.yml

@@ -63,7 +63,7 @@ steps:
     env:
       RUBY_VERSION: "3.4"
       STACK_VERSION: 8.18.0-SNAPSHOT
-      ES_YAML_TESTS_BRANCH: 8.x
+      ES_YAML_TESTS_BRANCH: 8.18
       TRANSPORT_VERSION: "8.3"
       RUBY_SOURCE: "ruby"
       TEST_SUITE: "platinum"

.github/workflows/otel.yml

@@ -2,10 +2,10 @@ name: opentelemetry
 on:
   push:
     branches:
-      - 8.x
+      - 8.18
   pull_request:
     branches:
-      - 8.x
+      - 8.18
 jobs:
   test-otel:
     name: 'Test Open Telemetry'