refactor: veYFI time

[xgambitox]

Description

• Refactor internal usage of weeks in favor of seconds as time units
• Display a more conventional time format

Related Issue

• https://github.com/yearn/yearn-sdk/pull/322

Motivation and Context

• Allow more granular units avoiding issues when rounding.

[aws-amplify-us-east-1[bot]]

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