Interactivity API: Remove data-wp-interactive object for core/router.

[cbravobernal]

In ​https://github.com/WordPress/wordpress-develop/commit/6345e2494793cad4294b792fc6ddc54a2bdeb94d , was added the support of
data-wp-interactive="myPlugin" apart from data-wp-interactive='{"namespace":"myPlugin"}'

In order to keep consistency with the rest of the blocks, that were updated in
​https://github.com/WordPress/gutenberg/pull/58943, and being Core as a guide for standards, this ticket updates the core/router namespace accordingly.

Trac ticket: https://core.trac.wordpress.org/ticket/60542

Gutenberg backport: ​https://github.com/WordPress/gutenberg/pull/59030

What was the original ticket where the same functionality landed? It would be better to link it instead when refactoring code unless it's a bug. Still, we would need the original commit and ticket to reference in the commit message.

The code that implements that change is part of the interactivity package, that landed in 17.7

• ​https://github.com/WordPress/gutenberg/pull/58743/

Was implemented here:

• ​https://github.com/WordPress/wordpress-develop/pull/6077/

And commited in this ticket:

• 60315

[swissspidy]

In 57646:

Interactivity API: Use string instead of object in data-wp-interactive attribute.

The server directive processing, integrated in [57563], supports a simplified format for passing the namespace to data-wp-interactive.

Props cbravobernal, gziolo.
Fixes #60542.
See #60356.

Merged in https://core.trac.wordpress.org/changeset/57646