Footer credit: Remove customizer option for block themes (take 2) (#3…

…8559)

Reintroduces the removal of the "Footer Credit" option from the Customizer for block themes (Automattic/jetpack#38473) which was reverted in Automattic/jetpack#38557 since it caused fatal errors on sites with child themes.

To prevent the fatal errors, we know wait for the `init` action before checking `wp_is_block_theme`.

Props to @Automattic/lego for finding the culprit.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/10108305659

Upstream-Ref: Automattic/jetpack@04a2033