change: ETCM-12424 remove dependency on slots from pallet-block-producer-fees

[AmbientTea]

Description

Replaces use of Slot in the pallet by a generic Moment parameter instead, leaving the chain builders to decide how they should identify fee change times.

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• The size limit of 400 LOC isn't needlessly exceeded
• The PR refers to a JIRA ticket (if one exists)
• New tests are added if needed and existing tests are updated.
• New code is documented and existing documentation is updated.
• Relevant logging and metrics added
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff