Planner component should be updated to the latest Fluent UI design

[sebastienlevert]

Figma file here

Hello sebastienlevert, thank you for opening an issue with us!

I have automatically added a "needs triage" label to help get things started. Our team will analyze and investigate the issue, and escalate it to the relevant team if possible. Other community members may also look into the issue and provide feedback 🙌

[gavinbarron]

@sebastienlevert I'm moving this to v3.1 milestone to acknowledge that we're blocked on this one and it won't be in 3.0