parse-datetime: avoid "%pure-parser" deprecation warning from Bison 3.4
* lib/parse-datetime.y: Use "%define api.pure" rather than obsolescent
"%pure-parser". The former is available since Bison 2.3b (2008, [1]),
while the latter is marked as obsolete since version 3.4 (May 2019, [2]):
./lib/parse-datetime.y:568.1-12: warning: deprecated directive, \
use '%define api.pure' [-Wdeprecated]
568 | %pure-parser
| ^~~~~~~~~~~~
[1] https://git.sv.gnu.org/cgit/bison.git/commit/?id=
d9df47b656fd
[2] https://git.sv.gnu.org/cgit/bison.git/commit/?id=
de5207244b5c