ci: move from dependabot reviewers to CODEOWNERS

Author: oakrizan

.github/CODEOWNERS

@@ -1 +1,5 @@
+# Default owner
+* @elastic/apm-agent-ruby
+
+# Sub-directories/files ownership
 /.github/workflows @elastic/observablt-ci

.github/dependabot.yml

@@ -10,8 +10,6 @@ updates:
       interval: "weekly"
       day: "sunday"
       time: "22:00"
-    reviewers:
-      - "elastic/apm-agent-ruby"
 
   # Enable version updates for Docker
   - package-ecosystem: "docker"
@@ -21,8 +19,6 @@ updates:
       interval: "weekly"
       day: "sunday"
       time: "22:00"
-    reviewers:
-      - "elastic/apm-agent-ruby"
 
   # GitHub actions
   - package-ecosystem: "github-actions"