MM-36271: Bump major version to 6.0 (#17973)

https://mattermost.atlassian.net/browse/MM-36271

```release-note
We bump the major version to 6.0
```

app/plugin.go

@@ -20,13 +20,13 @@ import (
 	svg "github.com/h2non/go-is-svg"
 	"github.com/pkg/errors"
 
-	"github.com/mattermost/mattermost-server/v5/app/request"
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/plugin"
-	"github.com/mattermost/mattermost-server/v5/services/marketplace"
-	"github.com/mattermost/mattermost-server/v5/shared/filestore"
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
-	"github.com/mattermost/mattermost-server/v5/utils/fileutils"
+	"github.com/mattermost/mattermost-server/v6/app/request"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/plugin"
+	"github.com/mattermost/mattermost-server/v6/services/marketplace"
+	"github.com/mattermost/mattermost-server/v6/shared/filestore"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/utils/fileutils"
 )
 
 const prepackagedPluginsDir = "prepackaged_plugins"