Fix _load_textdomain_just_in_time notice on WordPress 6.7+

[trajche]

Summary

• Replaces load_textdomain() with load_plugin_textdomain() in acf_load_textdomain() for proper integration with the just-in-time translation loading system introduced in WordPress 6.7.
• This fixes the _load_textdomain_just_in_time was called incorrectly notice that appears on WordPress 6.7+ installations.

Fixes #252

Test plan

• Install SCF on WordPress 6.7+ with WP_DEBUG and WP_DEBUG_DISPLAY enabled
• Verify no _load_textdomain_just_in_time notice appears for the secure-custom-fields domain
• Verify translations still load correctly on localized WordPress installations

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[trajche]

This was not an issue with SCF. It was caused by other plugins registering options page/acf fields too early, which initalized SCF. By wrapping it in acf/init action, it works just fine.