.gitattributes: Highlight Jenkinsfile as Groovy

See [1] for the documentation of those overrides.

1 - https://github.com/github/linguist/blob/master/docs/overrides.md
Signed-off-by: Paul Chaignon <paul@cilium.io>

.gitattributes

@@ -1,4 +1,5 @@
 /install/kubernetes/cilium/values.yaml.tmpl linguist-language=yml
+*.Jenkinsfile linguist-language=Groovy
 go.sum linguist-generated
 examples/kubernetes/connectivity-check/connectivity-*.yaml linguist-generated
 pkg/k8s/apis/cilium.io/v2/client/crds/*.yaml linguist-generated