ci: use reusable LibreCode info.xml lint workflow

[vitormattos]

Summary

Replace the copied Nextcloud lint-info-xml workflow implementation with the thin LibreCode caller introduced in LibreCodeCoop/github-workflows#22.

The consumer now owns only the pull_request trigger and delegates the job to the reusable workflow pinned at an immutable commit SHA.

Benefits:

• implementation fixes stay centralized;
• the consumer no longer duplicates checkout/schema/xmllint steps;
• the reusable implementation uses checkout v7.0.1;
• sparse checkout is limited to appinfo/;
• the Nextcloud App Store schema is pinned to an immutable commit rather than master.

This is the first consumer validation of the reusable-workflow distribution model from LibreCodeCoop/github-workflows#16.