Add the vendor directory to .gitignore

* gardener#748 removed the `vendor` directory from `etcd-druid` and makes the
  project rely on the module cache set up by the `go` tool for finding
  dependencies.

* However, there are use cases where for better developer experience, it
  would make sense to have the dependencies reside in the same project
  folder as `etcd-druid`, so that tokens can be searched for, using
  project wide search in any text editor. Results can be fetched from
  the dependencies, without having to rely on the LSP.
  For example, this would make looking at all usages of a constant that
  is defined by a dependency easier.

* The developer can simply run `go mod vendor` without it affecting the
  modified build process which doesn't use `vendor`, as defined in gardener#748.

.gitignore

@@ -4,6 +4,7 @@
 /.kube-secrets
 /tmp/*
 /dev
+/vendor
 *.test
 
 *.coverprofile
@@ -13,6 +14,7 @@
 .vscode
 .idea
 .DS_Store
+.ignore
 *~
 
 ./etcd-druid
@@ -23,6 +25,7 @@ config.img
 config.vmdk
 
 hack/e2e-test/infrastructure/overlays/gcp/common/assets
+hack/e2e-test/infrastructure/kind/kubeconfig
 
 /test/integration/resources/charts/*
 /test/integration/resources/repository/cache
@@ -34,6 +37,3 @@ hack/e2e-test/infrastructure/overlays/gcp/common/assets
 
 .cache_ggshield
 .gitguardian.yaml
-
-hack/e2e-test/infrastructure/kind/kubeconfig
-