[FEATURE] Make "text" message format workaround backwards compatible

[AWSHurneyt]

Is your feature request related to a problem?
As described in issue #818, custom webhooks do not support webhooks that do not take the message string directly as the request body. For webhooks like Google chat (documentation link), users create the channel using a Slack config, instead of a custom webhook config, because Slack uses a similar { text: "message content" } request body format.

Validation is currently being added for future releases which could cause issues when customers attempt to edit or migrate those workaround channels to future versions (e.g., PR #814).

What solution would you like?
Users should be able to seamlessly migrate their channels that used this workaround from versions without the config validation to versions with the validation, and should also be able edit those channels without issue.