Updates old-style copyright headers to the new style, by removing "All
rights reserved." Also inserts "The" before "PDFium Authors", which is
required by both styles.

Mechanically generated by this command:
  PATTERN='Copyright \([0-9]\+\) \(The \)\?PDFium Authors. All rights reserved.'
  git grep -l "$PATTERN" \
  | xargs sed "s/$PATTERN/Copyright \1 The PDFium Authors/" -i''

Fixed: pdfium:1884
Change-Id: I6f781d811df8839e2e21b85f716529b813828bcd
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/100371


Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: K. Moon <kmoon@chromium.org>