Run yamllint with strict mode, fix issue (go-gitea#33551)

Previously yamllint would issue warnings for certain things, while still
exiting with zero. Now warnings are treated like errors and will cause
non-zero exit:

```
  -s, --strict          return non-zero exit code on warnings as well as errors
```

(cherry picked from commit b330274)

Author: silverwind

Makefile

@@ -534,7 +534,7 @@ lint-templates: .venv node_modules
 
 .PHONY: lint-yaml
 lint-yaml: .venv
-	@poetry run yamllint .
+	@poetry run yamllint -s .
 
 .PHONY: security-check
 security-check: