port: remove Cortana from Channels enum (#3730)

[stevengum]

The changes in fix: remove Cortana from Channels enum (#3730) may need to be ported to maintain parity with microsoft/botbuilder-js.

Fixes microsoft/botbuilder-js#3603

Targeting cloud-adapter to avoid future merge conflicts

Specific Changes

• Remove Cortana from Channels enum in botframework-schema, see https://github.com/microsoft/botbuilder-js/issues/3603#issuecomment-856311472
• Hidden public helper hasMessageFeed() always returns true (no change on visibility in declarations)
• supportsCardActions() no longer has Channels.Cortana case (will use default false for 'cortana' channelId).
• remove/update Cortana specific tests in choices_channel.test.js and choices_choiceFactory.test.js
• OAuthPrompt.channelSupportsOAuthCard() no longer has Channels.Cortana case (will use default true for 'cortana' channelId).

Please review and, if necessary, port the changes.