port: Add Obsolete attribute to custom adapters. (#6137)

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The changes in Add Obsolete attribute to custom adapters. (#6137) may need to be ported to maintain parity with microsoft/botbuilder-dotnet.

Fixes #5976

Description

This change adds the Obsolete attribute to the custom adapters. The change that adds the adapters to the Community repo can be found here: botbuilder-community-dotnet PR 461

Please review and, if necessary, port the changes.