fix: document naming rule not working for subscription invoices (backport #27386)

[mergify]

Document Naming Rule on Sales Invoice

Created a Subscription for the Company associated with that Company GSTIN

Now clicking on Fetch Subscription Updates on the Subscription document creates a Sales Invoice with default naming instead of following the Document Naming Rule.

This is because the Company GSTIN is set in the validate method of Sales Invoice while the naming is set before validate, hence while setting the name, the Company GSTIN condition is false

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This is an automatic backport of pull request #27386 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 9c27f9b has failed:

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	both modified:   erpnext/accounts/doctype/subscription/subscription.py

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