Update docker.io/openproject/openproject Docker tag to v15.2.1

.woodpecker/.build.yaml

@@ -1,5 +1,3 @@
-labels:
-  backend: kubernetes
 steps:
 - name: docker
   image: woodpeckerci/plugin-docker-buildx

.woodpecker/.deploy.yaml

@@ -1,6 +1,6 @@
 skip_clone: true
 labels:
-  backend: kubernetes
+  backend: docker
 steps:
 - name: bump tag in deployment-repo
   image: git.ar21.de/aaron/debian-ci

.woodpecker/.lint.yaml

@@ -1,5 +1,5 @@
 labels:
-  backend: kubernetes
+  backend: docker
 steps:
 - name: Lint Dockerfile
   image: hadolint/hadolint:latest-alpine

.woodpecker/.test.yaml

@@ -1,6 +1,6 @@
 skip_clone: true
 labels:
-  backend: kubernetes
+  backend: docker
 steps:
 - name: Test image (prod)
   image: git.ar21.de/${CI_REPO}:${CI_PIPELINE_NUMBER}
@@ -9,7 +9,7 @@ steps:
   when:
     - branch: main
       event: [push, manual]
-- name: Test image (testing)
+- name: Test image
   image: git.ar21.de/${CI_REPO}:testing-${CI_PIPELINE_NUMBER}
   commands:
     - echo test