fix handling of backport changelog

.github/update-release-branch.py

@@ -187,14 +187,18 @@ def process_changelog_for_backports(source_branch_major_version, target_branch_m
 
     # until we find the first section, just duplicate all lines
     while True:
+      found_first_section = False
       line = f.readline()
       if not line:
         raise Exception('Could not find any change sections in CHANGELOG.md') # EOF
 
-      output += line
       if line.startswith('## '):
         line = line.replace(f'## {source_branch_major_version}', f'## {target_branch_major_version}')
-        # we have found the first section, so now handle things differently
+        found_first_section = True
+
+      output += line
+      if found_first_section:
+        # we now handle things differently
         break
 
     # found_content tracks whether we hit two headings in a row