Disable MM Ingress by default

See upstream PR: https://github.com/mattermost/mattermost-operator/pull/249

New upstream changes introduced the ability to disable the MM ingress by default with spec.ingress.enabled = false. Update the default Mattermost spec to reflect this.

AC:

• Mattermost spec updated to default ingress to disabled
• Tag bumped + gatekeeper exception removed in BB MR - https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/master/chart/templates/gatekeeper/values.yaml#L69-76

NOTE: Security because it touches on Gatekeeper/disabling unused ingress is security :)