fix: slot reservation uses wrong event length

[CarinaWolli]

What does this PR do?

Fixes that for events with multiple duration, the slot reservation system uses the wrong event length to reserve slots.

Requirement/Documentation

Type of change

• Bug fix (non-breaking change which fixes an issue)

How should this be tested?

• create event type with multiple durations choose 15 min and 120 min
• make 120 min the default duration
• go to the event type link
• pick any 15min slot (for example 11:00-11:15) => this slot is reserved now
• Open a new browser tab with the event-type link
• check available slots for 15 min duration at the day you booked the event:
  • expected slots: all slots except 11:00 should be here
  • before: all slots between 9:00 and 11:00 were missing

[vercel]

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Name	Status	Preview	Updated (UTC)
api	✅ Ready (Inspect)	Visit Preview	Jul 12, 2023 5:44pm
cal	✅ Ready (Inspect)	Visit Preview	Jul 12, 2023 5:44pm
ui	✅ Ready (Inspect)	Visit Preview	Jul 12, 2023 5:44pm

[github-actions]

Thank you for following the naming conventions! 🙏

[alwaysmeticulous]

🤖 Meticulous spotted visual differences in 71 of 155 screens tested: view and approve differences detected.

_{Last updated for commit 317651d. This comment will update as new commits are pushed.}

[github-actions]

📦 Next.js Bundle Analysis for @calcom/web

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌

[zomars]