Backport missing changes to WP 6.5 schema and docs

[ajlende]

What?

Fixes the 6.5 JSON schema and docs. There were some last minute changes for 6.5 that didn't get backported for the theme.json schema.

Why?

Bug fix / backport. Some schema changes were discovered to be missing in #61221. The theme.json schema on this branch is linked to https://schemas.wp.org/wp/6.5/theme.json and may be used by themes to verify that a theme.json works with this version of WordPress. Same for the block.json.

How?

• background.backgroundSize added in 6.5
  • Background image: Add backgroundSize and repeat features #57005
  • JSON schema: Update schema for background support #59127
  • Theme docs: Update appearance-tools docs to reflect opt-in for backgroundSize and aspectRatio #59165
• shadow.defaultPresets changed and then reverted in 6.5
  • Disable core shadow presets by default, let themes opt-in #58766
  • theme.json schema: Update appearanceTools description #59499
  • Shadow: revert shadow default presets opt-in to opt-out #60204

Testing Instructions

Confirm that these options are correct for 6.5

Testing Instructions for Keyboard

Screenshots or screencast

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Co-authored-by: ajlende <ajlende@git.wordpress.org>
Co-authored-by: fabiankaegy <fabiankaegy@git.wordpress.org>

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