[Slack integration] files.completeUploadExternal.initial_comment does not support mrkdwn format

[teka23]

https://github.com/looker-open-source/actions/blob/f9a87a9b67758271b9144770be6017992c61d30f/src/actions/slack/utils.ts#L143

It looks like files.completeUploadExternal.initial_comment does not support mrkdwn format and the comment is showing up as plain text.

[teka23]

https://github.com/slackapi/node-slack-sdk/issues/1561

[teka23]

As Slack people recommended, we might need to upload the file first and use that file as an attachment. Or, send comment first and add the attachment to the thread