Added a deprecation warning for the command dumper

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12620
| License       | MIT

This fixes #12620
Sorry, I didn't explain well enough: I meant modifying `->setDescription('Dumps all routes as Apache rewrite rules')` to 
` ->setDescription('[DEPRECATED] Dumps all routes as Apache rewrite rules')`
can you please do that?