Fix: avoid vendoring symfony packages already in core

[Rom1-B]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes #N/A
• The plugin's CI flagged symfony/deprecation-contracts and symfony/polyfill-ctype as conflicting with GLPI core's own copies.
• These are transitive dependencies of symfony/yaml, already shipped by core, so they are now declared under composer's provide and dropped from the plugin's own vendor tree.

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