Feat: update to 0.31.5 version of decidim

[Stef-Rousset]

🎩 Description

This PR updates decidim to 0.31.5, to fix an error on the CI of decidim rc-4.0.0 (Your bundle is locked to decidim-ai (0.31.2) from
https://github.com/OpenSourcePolitics/decidim-module-ai.git (at
bump/0.31_third_party_service@0b04b83), but that version can no longer be found
in that source. That means the author of decidim-ai (0.31.2) has removed it.
You'll need to update your bundle to a version other than decidim-ai (0.31.2)
that hasn't been removed in order to install.)

Testing

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📌 Related Issues

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• Fixes #?

Tasks

• Add specs
• In case of new dependencies or version bump, update related documentation

🛠️ If your PR introduces or updates an Override

• Add the label "override:pending" in your PR
• Add the corresponding label based on the review deadline category (when this override must be checked):
  • WHEN BACKPORT 0.29 ·
  • BEFORE MIGRATION 0.31 ·
  • WAITING FOR PR UPSTREAM ·
  • OTHER
• Add the link to the related Decidim issue or PR (if exist).

📷 Screenshots

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