Prior to this commit, all `WebMvcConfigurer` instances provided by user
configuration were processed *before* the one provided by the
`WebMvcAutoConfiguration`.

For many options this has no consequence, but for some, like the
`ContentNegotiationConfigurer`, settings were overriden by the
auto-configuration even if developers provided an opinion.

This commit orders the `WebMvcConfigurer` provided by the
auto-configuration at `0`, so that custom configurers (unordered, at
`Ordered.LOWEST_PRECEDENCE`) are processed *after*.

This still gives room to developers for configuring things *before* the
auto-configuration - they can still order their own configuration
accordingly.

Fixes gh-12389