Update mattermost/mattermost-enterprise-edition Docker tag to v6

This MR contains the following updates:

Package	Type	Update	Change
mattermost/mattermost-enterprise-edition	ironbank-docker	major	5.39.0 -> 6.0.0
mattermost/mattermost-enterprise-edition		major	5.39.0 -> 6.0.0
mattermost/mattermost-enterprise-edition	stage	major	5.39.0 -> 6.0.0

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