Update Web Chat to 4.15.2

[compulim]

botframework-webchat@4.15.2 is using adaptivecards@2.10.0, which is expected to parse up to AC schema 1.5.

Microsoft Reviewers: Open in CodeFlow

[opbld32]

Docs Build status updates of commit fd249ea:

✅ Validation status: passed

File	Status	Preview URL	Details
AdaptiveCards/resources/partners.md	✅Succeeded

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