Deprecate route `format`, replacing with `pattern`

To make the code easier to understand and to support trie-based routing for #3328, it would be helpful to replace the several remaining instances of route `format`s with `patterns`.

Many of the remaining `format`s are designed to support aliases, and since we've started using redirects heavily, the likely solution is to rewrite the canonical form using a pattern, and add redirects for each of the alias forms.

Ideally by using multiple redirects we can rewrite everything using patterns.

Start by locating the remaining route definitions that use format and inspect how their aliases and canonical forms are represented. The work is done when canonical routes use patterns and each supported alias is handled through redirects, while preserving existing routing behavior.

Written by the indexing model from the issue text.