Update Woodpecker config for version 1

- `pipeline:` renamed to `steps:`
- changed file extension from `yml` to `yaml`, to align with other files

For more info, see <https://woodpecker-ci.org/docs/next/migrations>

.woodpecker.yaml

@@ -3,7 +3,7 @@ variables:
   - &create_synapse_access_token >-
     echo '{"user_id":"ci-dummy","access_token":"ci-dummy","home_server":"ci-dummy","device_id":"ci-dummy"}' > src/config/synapse_access_token.json
 
-pipeline:
+steps:
   lint-markdown:
     image: markdownlint/markdownlint:latest
     group: test