ci: replace deprecated workflow syntax

.woodpecker/docs.yml

@@ -8,13 +8,13 @@ when:
 steps:
   - name: markdownlint
     image: quay.io/thegeeklab/markdownlint-cli
-    group: test
+    depends_on: []
     commands:
       - markdownlint 'README.md'
 
   - name: spellcheck
     image: quay.io/thegeeklab/alpine-tools
-    group: test
+    depends_on: []
     commands:
       - spellchecker --files 'README.md' -d .dictionary -p spell indefinite-article syntax-urls
     environment:
@@ -22,6 +22,6 @@ steps:
 
   - name: link-validation
     image: docker.io/lycheeverse/lychee
-    group: test
+    depends_on: []
     commands:
       - lychee --no-progress --format detailed README.md