Backport commit 946a56a10 from Emacs

* lisp/org.el (org-doi-server-url): Prefer https: to http: un URLs.

Update some URLs
946a56a10fed769646a8b7c4ebc53f53c84be896
Paul Eggert
Mon Sep 23 00:12:52 2019 -0700

Notes(km):

  - It's not mentioned in 946a56a10's message, but that commit also
    drops the dx subdomain, an "earlier syntax which continues to be
    supported" according to https://www.doi.org/factsheets/DOI_PURL.html.

  - 946a56a10 replaced a link in the manual with https://api.uva.nl/
    (which the previous link redirects to).  Here we instead use the
    replacement link from master's f4083eefd (manual: Fix URL,
    2019-06-01).

doc/org-manual.org

@@ -3160,7 +3160,7 @@ External links are URL-like locators.  They start with a short
 identifying string followed by a colon.  There can be no space after
 the colon.  The following list shows examples for each link type.
 
-| =http://www.astro.uva.nl/=dominik=        | on the web                     |
+| =https://staff.science.uva.nl/c.dominik/= | on the web                     |
 | =doi:10.1000/182=                         | DOI for an electronic resource |
 | =file:/home/dominik/images/jupiter.jpg=   | file, absolute path            |
 | =/home/dominik/images/jupiter.jpg=        | same as above                  |

lisp/org.el

@@ -2378,7 +2378,7 @@ For more examples, see the system specific constants
 			(string :tag "Command")
 			(function :tag "Function")))))
 
-(defcustom org-doi-server-url "http://dx.doi.org/"
+(defcustom org-doi-server-url "https://doi.org/"
   "The URL of the DOI server."
   :type 'string
   :version "24.3"