Update TOC.yml with Viva Engage

[danthemango]

Rename Yammer to Viva Engage in TOC
to reflect updated branding

This change is related to commit d2ac96c which failed to update the TOC.

[learn-build-service-prod]

Learn Build status updates of commit e1e522c:

✅ Validation status: passed

File	Status	Preview URL	Details
microsoft-365/enterprise/TOC.yml	✅Succeeded

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[danthemango]

I'm unsure if we should also update the links too, they currently redirect to https://learn.microsoft.com/en-us/viva/engage/overview but based on the context (deploy, manage) I believe urls should be update to something like https://learn.microsoft.com/en-us/viva/engage/setup and https://learn.microsoft.com/en-us/viva/engage/eac-overview