Remove deprecated Gutenberg Fonts API workaround

[dd32]

Summary

• Remove the inject_fonts_theme_json method and its wp_theme_json_data_theme filter from the wc-fonts plugin
• This was a temporary workaround (with a @todo to remove it when Gutenberg 17.6 was rolled out) for injecting custom fonts before the core Font Library was available
• Gutenberg is now at 22.6, well past the 17.6 milestone, so this workaround is no longer needed
• The hardcoded fonts (Krona One, Lora for WCEU 2024; Roboto, Roboto Slab, Sansita for WC Asia 2024) were for past events
• Also removes usage of _wp_array_get and _wp_array_set which are deprecated since WP 6.7

Fixes #1549

Test plan

• Verify no deprecated font notices are thrown on WordCamp sites
• Verify the Appearance > Fonts settings page still works for Typekit, Google Web Fonts, Font Awesome, and Dashicons
• Verify the Font Library is accessible and functional for adding custom fonts to WordCamp sites

Generated with Claude Code

[dd32]

Note: Verify #1075 and #1216 first, verify that the fonts are re-enabled/imported onto the sites in question after merge, etc.