Deprecate the action and introduce filter

[Debarghya-Banerjee]

Trac Ticket: Core-52721

Summary

This PR introduces a change to the wp_signon function, deprecating the wp_authenticate action and replacing it with a filter, wp_signon_credentials.

Key Changes

• Deprecated the wp_authenticate action and added the new wp_signon_credentials filter.

Why This Change

• The shift from the wp_authenticate action to a filter is driven by the need for more flexible and streamlined handling of login credentials. The action wp_authenticate only allowed for limited modification of the credentials, and its callback structure was not as versatile as using a filter.

• The wp_authenticate action is a legacy mechanism that lacks flexibility for modern development. This PR deprecates it and introduces the wp_signon_credentials filter, offering a more extensible and standardized way to modify sign-on credentials. This change improves control over the login process, aligns with best practices, and ensures cleaner, more maintainable code. It also addresses the @todo from [27353], which questioned the deprecation of the action, by replacing it with the filter for improved flexibility while maintaining backward compatibility.

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