Path traversal for local publishers in TechDocs backend

Summary

### Impact A malicious actor with the ability to register entities in the Software Catalog is able to write files to arbitrary paths on the techdocs backend host instance when `techdocs.publisher.type` is set to `local`.

This vulnerability is mitigated by the fact that the Software Catalog must be configured with non-standard field format validators and/or non-standard entity policies.

### Patches Those affected are advised to upgrade to `@backstage/plugin-techdocs-node` version `1.1.2` or higher.

### Workarounds If patching or upgrading is not possible, it would be sufficient to update any custom Catalog field format validators and/or custom entity policies to disallow entity names, kinds, and namespaces containing `..`

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