opt: use method to obtain miner sector expiration

[simlecode]

Related Issues

Proposed Changes

use method GetMinSectorExpiration to obtain miner sector expiration

Additional Info

Checklist

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• PR title is in the form of of <PR type>: <area>: <change being made>
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[Stebalien]

Code LGTM but lints are failing.

[ribasushi]

cc @magik6k whether still relevant

[rvagg]

pulled in via #12376