update http links

- prefer https

- metadataworkinggroup.org/com seem to be offline; the web archive link
  was obtained from exiftool: https://exiftool.org/TagNames/MWG.html

- fix kramdown link, rubyforge has been gone a long time

- fix png/zlib links

Bug: webp:544
Bug: b/202302177
Change-Id: Id69de4553e7baf00393f12a2c1acb262443a1a93

man/gif2webp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH GIF2WEBP 1 "May 1, 2020"
+.TH GIF2WEBP 1 "November 17, 2021"
 .SH NAME
 gif2webp \- Convert a GIF image to WebP
 .SH SYNOPSIS
@@ -131,7 +131,7 @@ Please report all bugs to the issue tracker:
 https://bugs.chromium.org/p/webp
 .br
 Patches welcome! See this page to get started:
-http://www.webmproject.org/code/contribute/submitting-patches/
+https://www.webmproject.org/code/contribute/submitting\-patches/
 
 .SH EXAMPLES
 gif2webp picture.gif \-o picture.webp
@@ -160,5 +160,5 @@ Debian project (and may be used by others).
 .BR dwebp (1),
 .BR webpmux (1)
 .br
-Please refer to http://developers.google.com/speed/webp/ for additional
+Please refer to https://developers.google.com/speed/webp/ for additional
 information.