scripts/i18n: also normalize casing of UTF-8 in Content-Type

f626260a376c introduced invariant msgmerge casing. Do the same when normalizing
to ensure consistency also without msgmerge and to avoid unnecessary conflicts.

scripts/i18n_utils.py

@@ -133,6 +133,9 b' def _normalize_po(raw_content):'
             .strip(),
         '',
         header_normalize_re.sub('', raw_content[header_start:header_end])
+            .replace(
+                r'"Content-Type: text/plain; charset=utf-8\n"',
+                r'"Content-Type: text/plain; charset=UTF-8\n"')  # maintain msgmerge casing
             .strip(),
         '']  # preserve normalized header
     # all chunks are separated by empty line