Enable merge groups for conflicting-pr-label workflow

Adds merge_group trigger to the conflicting-pr-label workflow to enable
automatic merge group handling, consistent with other CI workflows in
this repository (codeql-analysis, gradle-build, sonarcloud, api-level-lint).

This change eliminates the need to manually rebase PRs when using merge
queues.

Adapted from microsoft/kiota#6952

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: ramsessanchez

.github/workflows/conflicting-pr-label.yml

@@ -11,6 +11,7 @@ on:
   pull_request:
     types: [synchronize]
     branches: [main, support/2.x.x]
+  merge_group:
 
 # A workflow run is made up of one or more jobs that can run sequentially or in parallel
 jobs: