app: Fix deprecation warning
app: Fix deprecation warning

The "new way" is supported since at least 2010 (!).

app/parse-datetime.y:302.1-12: warning: deprecated directive: ‘%pure-parser’, use ‘%define api.pure’ [-Wdeprecated]
  302 | %pure-parser
      | ^~~~~~~~~~~~
      | %define api.pure

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak