fix: use posting_date instead of bill_date from purchase invoice (backport #50772)

[mergify]

Issue: During asset creation if the purchase Invoice is linked, system sets Supplier Invoice Date as the Purchase Date instead of Posting Date.

Ref: 54340

Backport Needed: Version-15

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This is an automatic backport of pull request #50772 done by Mergify.

[mergify]

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	both modified:   erpnext/assets/doctype/asset/asset.py

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