Replace composer with tester (and use lists for plugins)

Author: toote

.buildkite/pipeline.yml

@@ -1,17 +1,19 @@
 steps:
   - label: ":bash: Plugin"
     plugins:
-      docker-compose#v4.9.0:
-        run: plugin
+      - plugin-tester#v1.0.0: ~
+
   - label: ":ruby: Ruby"
     plugins:
-      docker-compose#v4.9.0:
-        run: ruby
+      - docker-compose#v4.11.0:
+          run: ruby
+
   - label: "✨ Lint"
     plugins:
-      plugin-linter#v3.0.0:
-        id: junit-annotate
+      - plugin-linter#v3.1.0:
+          id: junit-annotate
+
   - label: ":bash: Shellcheck"
     plugins:
-      shellcheck#v1.3.0:
-        files: hooks/*
+      - shellcheck#v1.3.0:
+          files: hooks/*