"Pull request merged" notifications display the wrong time

[fmichaut-diff]

Describe the bug When a pull request is merged, the slack notification display the time the PR was created instead of the time the PR was merged : It looks like the PR got merged before it got approved...

To Reproduce

1. Open a pull request
2. Wait a few minutes
3. Merge the PR and observe the notification using the created time of the PR instead of the merged one

Expected behavior The time of the event ("Pull request merged by xxx") should be the one displayed on the notification, not the time the PR was created

[anuraag016]

This is the default behaviour, we show the PR created date for merge/closed/opened cards. Changing this would be a expectation change. cc: @ashokirla to take a call on this.