Explicitly deprecate WP_Debug_Data::get_sizes()

[SergeyBiryukov]

WP_Debug_Data::get_sizes() is unused in core as of [49154] (WordPress 5.6) and was soft-deprecated in [60272].

I think we can explicitly deprecate it by adding a _deprecated_function() call at the top:

_deprecated_function( __METHOD__, '5.6.0', 'WP_REST_Site_Health_Controller::get_directory_sizes()' );

Trac ticket: 63516
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This PR explicitly deprecates WP_Debug_Data::get_sizes() function according to suggestions in trac ticket (unused in core)

I've tested that deprecation works as intended by calling WP_Debug_Data::get_sizes() in a test plugin

Thanks!

Thanks for your first contribution 🎉
It works well, and the Depreciation info is shown as expected:
#### Screenshot:

[SergeyBiryukov]

In 60274:

Site Health: Explicitly deprecate WP_Debug_Data::get_sizes(), unused since WP 5.6.

Follow-up to [49154], [60272].

Props yashjawale, dhruvang21, ankitkumarshah, SergeyBiryukov.
Fixes #63516.