refactor: further rename --handler to --sort-by-handler for routes

[paulbalandan]

Description
Follow up to #10113 .

I realized --handler is quite ambiguous. It could mean the option is requiring a value (the handler) to be passed but in reality is a flag.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

We need to update user guide too: https://github.com/codeigniter4/CodeIgniter4/blob/develop/user_guide_src/source/incoming/routing.rst?plain=1#L1008