Update module github.com/mattermost/mattermost-server/v5 to v6

Author: renovate[bot]

go.mod

@@ -17,6 +17,7 @@ require (
 	github.com/labstack/echo/v4 v4.10.2
 	github.com/lestrrat-go/jwx/v2 v2.0.10
 	github.com/mattermost/mattermost-server/v5 v5.39.3
+	github.com/mattermost/mattermost-server/v6 v6.7.2
 	github.com/rubenv/sql-migrate v1.4.0
 	github.com/sirupsen/logrus v1.9.3
 	golang.org/x/oauth2 v0.8.0