golangci-lint: config syntax to exclude files has changed

this silences the following warning:
```
WARN [config_reader] The configuration option `run.skip-files` is deprecated, please use `issues.exclude-files`.
```

Author: joonas-fi

Diff for: .golangci.yml

@@ -1,6 +1,6 @@
 
-run:
-  skip-files:
+issues:
+  exclude-files:
     # golangci-lint doesn't recognize our generated files (*.gen.go)
     - ".+\\.gen\\.go$"