Prevent updating tournament start date to a past date in UpdateTournament mutation

[Dejmenek]

Problem

When updating a tournament via the UpdateTournament mutation, it is currently possible to change the start date to a date in the past (relative to now). This should not be allowed, as tournaments should only have start dates that are present or future-oriented.

Solution

• Add server-side validation to the UpdateTournament mutation.
• Prevent start date from being changed to any date that is in the past at the time of the mutation request.

Tasks

• Implement validation logic in UpdateTournament mutation.
• Update error messaging to inform the user if a past date is provided.
• Add test cases to ensure this validation works as intended (both positive and negative scenarios).