Update changelog branch name (ydb-platform#15188)

Author: galnat

.github/actions/update_changelog/update_changelog.py

@@ -204,7 +204,7 @@ def fetch_user_details(username):
 
     update_changelog(changelog_path, pr_data)
 
-    base_branch_name = f"dev-changelog-{base_branch}-{suffix}"
+    base_branch_name = f"changelog/{base_branch}-{suffix}"
     branch_name = base_branch_name
     index = 1
     while branch_exists(branch_name):