Upgrade dependency to timezone 0.8.0 to avoid using dependency_overrides

[Zelfapp]

While comment in #72 to use dependency_overrides in pubspec.yaml does allow one to continue to use flutter_contacts, I would suggest this issue stay open until fully resolved by upgrading or removing deprecated dependencies that flutter_contacts has.

dependency_overrides:
  timezone: 0.7.0

flutter_contacts relies on flexidate. flexidate is not compatible with timezone 0.8.0 and flutter_local_notfications 9.0.0 relies on timezone 0.8.0.

flutter_local_notfications is a very popular app and it makes flutter_contacts hard to use when we have to resort to using dependency overrides, which is not a good practice.