doc: Update RFC URLs

Consistently use the IETF HTML links instead of a random mix of
different sites and formats.  This also fixes one broken link for JSON
documentation.

doc/src/sgml/json.sgml

@@ -13,7 +13,7 @@
 
  <para>
   JSON data types are for storing JSON (JavaScript Object Notation)
-  data, as specified in <ulink url="http://rfc7159.net/rfc7159">RFC
+  data, as specified in <ulink url="https://tools.ietf.org/html/rfc7159">RFC
   7159</ulink>. Such data can also be stored as <type>text</type>, but
   the JSON data types have the advantage of enforcing that each
   stored value is valid according to the JSON rules.  There are also

doc/src/sgml/libpq.sgml

@@ -772,7 +772,7 @@ PGPing PQping(const char *conninfo);
     Several <application>libpq</> functions parse a user-specified string to obtain
     connection parameters.  There are two accepted formats for these strings:
     plain <literal>keyword = value</literal> strings
-    and <ulink url="http://www.ietf.org/rfc/rfc3986.txt">RFC
+    and <ulink url="https://tools.ietf.org/html/rfc3986">RFC
     3986</ulink> URIs.
    </para>
 

doc/src/sgml/pgcrypto.sgml

@@ -1329,15 +1329,15 @@ gen_random_uuid() returns uuid
 
    <itemizedlist>
     <listitem>
-     <para><ulink url="http://www.ietf.org/rfc/rfc4880.txt"></ulink></para>
+     <para><ulink url="https://tools.ietf.org/html/rfc4880"></ulink></para>
      <para>OpenPGP message format.</para>
     </listitem>
     <listitem>
-     <para><ulink url="http://www.ietf.org/rfc/rfc1321.txt"></ulink></para>
+     <para><ulink url="https://tools.ietf.org/html/rfc1321"></ulink></para>
      <para>The MD5 Message-Digest Algorithm.</para>
     </listitem>
     <listitem>
-     <para><ulink url="http://www.ietf.org/rfc/rfc2104.txt"></ulink></para>
+     <para><ulink url="https://tools.ietf.org/html/rfc2104"></ulink></para>
      <para>HMAC: Keyed-Hashing for Message Authentication.</para>
     </listitem>
     <listitem>