staging: Deploy swh.loader.metadata and friends

Deployments:

• loader-core v3.2.0 (for CLI + fork support)
• loader-git v1.6.0 (fork support)
• loader-metadata v0.0.2 (fork support)

Plan:

• Upgrade those packages on staging workers
• Restart swh-worker@loader_git (ongoing)
• Restart swh-worker@loader_high_priority
• Check it's behaving appropriately

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Migrated from T4202 (view on Phabricator)