Packages and Tools: Ensure proper flags are used with json_encode() (#46092)

This is a best-effort application of proper flags for `json_encode()`
and `wp_json_encode()`.

It also includes some cleanup of PHP 5.2 compatibility
(`JSON.parse(decodeURIComponent())`), which was a workaround for when
the flags we now use did not exist.

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Co-authored-by: Brad Jorsch <[email protected]>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/19897898313

Upstream-Ref: Automattic/jetpack@12105b2

Author: anomiex