Fix regression with theme.json settings.shadow.defaultPresets (minimal)

[ajlende]

All discussion for this issue should be done in WordPress/gutenberg#59989

Alternative to #6295 without the editor-shadow-presets theme support feature.

• Reverts core theme.json settings.shadow.defaultPresets to true to fix the regression. (Back to working the same as color/gradient preset configuration for block themes.)

The color/gradient support behavior that I think we should be matching enables support for the presets in classic themes by default and allows themes to opt-out. For example, gradients can be opted-out of by setting add_theme_support( 'editor-gradient-presets', array() ).

If we're okay with shadows being opt-out in the UI like colors/gradients, we can merge just this and add the ability to opt-out with add_theme_support( 'editor-shadow-presets', array() ) that's included in #6295 as a follow-up in 6.6.

Trac ticket: https://core.trac.wordpress.org/ticket/60815

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Props ajlende.

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[swissspidy]

Looks like we're converging towards focusing on #6309, so closing this one.