FIX: Ensure upgrader_process_complete fires for bulk updates

[patil-vipul]

Summary

The upgrader_process_complete hook currently only fires for individual plugin updates but is skipped during bulk operations due to an is_multi condition check. This inconsistency breaks plugins that rely on post-upgrade actions like cache clearing, security scans, and configuration updates.

Changes Made

• Move the upgrader_process_complete hook from the is_multi condition in WP_Upgrader::run()
• Keep the is_multi check only for $this->skin->footer() calls
• Maintain existing bulk upgrade hooks in Plugin_Upgrader and Theme_Upgrader for backward compatibility

Trac ticket: https://core.trac.wordpress.org/ticket/63596

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[github-actions]

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[patil-vipul]

@SergeyBiryukov Can you look at this change, thanks 🙇 .