Log Message:
libidn2: update to 2.3.1.

* Noteworthy changes in release 2.3.1 (2021-05-12) [stable]

** Implement full roundtrip for lookup functionality
With TR64 enabled (default), '☺i' was converted to 'xn-- o-oia59s'.
The output contains an illegal space and thus could not be decoded any
more.  Reported-by: Chris Malton.  See
<https://gitlab.com/libidn/libidn2/issues/78> and
<https://gitlab.isc.org/isc-projects/bind9/issues/1610>.

** Fix domain too long error

** doc: idn2.1 and libidn2.texi automatically get idn2 --help output.

** Updated gnulib files and various build fixes.
In particular, it no longer attempts to detect a host CC compiler.
See <https://gitlab.com/libidn/libidn2/-/issues/83>.