Block Library: Navigation: Refactor to use render_callback

[aduth]

Previously: #21467, #19991

Context:

#21467 enables support for render_callback to receive the block instance as an argument. This had been needed for the Navigation block in referencing inner blocks, originally implemented in #16796 as an extra argument of render_callback, proposed for core enhancement at Trac#48104, then ultimately reverted to an alternative (admittedly more cumbersome) approach using filtering in #19991.

Task:

Revise the implementation of Navigation block to use render_callback, using the $block argument to reference inner blocks.

Motivation:

This is largely for code quality and maintainability of the block, since using the render_block filter is an unnecessarily complex means to customize the render output of a block, vs. the first-class render_callback callable setting of a block.

[talldan]

I can handle this as part of #21075.

[talldan]

Closed by #21075