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
```

plugin/checker/check_api.go

@@ -8,8 +8,8 @@ import (
 	"go/ast"
 	"go/token"
 
-	"github.com/mattermost/mattermost-server/v5/plugin/checker/internal/asthelpers"
-	"github.com/mattermost/mattermost-server/v5/plugin/checker/internal/version"
+	"github.com/mattermost/mattermost-server/v6/plugin/checker/internal/asthelpers"
+	"github.com/mattermost/mattermost-server/v6/plugin/checker/internal/version"
 )
 
 func checkAPIVersionComments(pkgPath string) (result, error) {