Pin Gutenberg plugin version and address test issues

[oguzkocer]

Summary

• Pin Gutenberg plugin version to 21.7.0 for test server
• Fix PostTypeSupports deserialization to handle empty arrays
• Fix Template.theme field to be properly optional

Changes

• Modified test server setup to download and install Gutenberg 21.7.0 directly from WordPress.org
• Wrapped PostTypeSupports in PostTypeSupportsMap with custom deserialization to handle cases where WordPress returns [] instead of {}
• Made Template.theme field properly optional with #[WpContextualOption] attribute
• Updated Kotlin and Swift integration tests to access supports.map instead of supports directly