Update `intl` to ^0.18.0

[iqfareez]

Error when running flutter pub get

Because ical >=0.1.4 depends on intl ^0.17.0 and my_app depends on intl ^0.18.0, ical >=0.1.4 is forbidden.
So, because my_app depends on ical ^0.2.2, version solving failed.
pub get failed

[lukas-h]

Hey, I would appreciate a PR updating the versions. Thanks for raising the issue

[iqfareez]

@lukas-h PR ready