Update mattermost/mattermost-operator Docker tag to v1.16.0

This MR contains the following updates:

Package	Type	Update	Change
mattermost/mattermost-operator	ironbank-docker	minor	v1.15.0 -> v1.16.0
mattermost/mattermost-operator		minor	v1.15.0 -> v1.16.0
mattermost/mattermost-operator	stage	minor	v1.15.0 -> v1.16.0

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