minor #14402 Update ICU project guide website (SiM07)

This PR was merged into the 3.4 branch.

Discussion
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Update ICU project guide website

Follow discussion about this subject in twig doc twigphp/Twig#3409

Commits
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7a89d40 Update ICU project doc website

Author: wouterj

reference/forms/types/datetime.rst

@@ -222,4 +222,4 @@ Field Variables
 +--------------+------------+----------------------------------------------------------------------+
 
 .. _`datetime local`: http://w3c.github.io/html-reference/datatypes.html#form.data.datetime-local
-.. _`Date/Time Format Syntax`: http://userguide.icu-project.org/formatparse/datetime#TOC-Date-Time-Format-Syntax
+.. _`Date/Time Format Syntax`: https://unicode-org.github.io/icu/userguide/format_parse/datetime/#datetime-format-syntax

reference/forms/types/options/date_format.rst.inc

@@ -29,6 +29,6 @@ For more information on valid formats, see `Date/Time Format Syntax`_::
     (the `RFC 3339`_ format) which is the default value if you use the
     ``single_text`` widget.
 
-.. _`Date/Time Format Syntax`: http://userguide.icu-project.org/formatparse/datetime#TOC-Date-Time-Format-Syntax
+.. _`Date/Time Format Syntax`: https://unicode-org.github.io/icu/userguide/format_parse/datetime/#datetime-format-syntax
 .. _`IntlDateFormatter::MEDIUM`: https://php.net/manual/en/class.intldateformatter.php#intl.intldateformatter-constants
 .. _`RFC 3339`: https://tools.ietf.org/html/rfc3339