Fixes#40031
While removing deprecated non-symbol access to nested `config_for`
hashes in #37876, we also broke `config_for` for anyone using the
[safe_yaml] gem. The problem is that `safe_yaml` patches `YAML.load` in
a way that doesn't honor the `symbolize_names` options (I believe this
is on purpose, to prevent symbol-based DOS attacks).
In the description of #37876 there is mention of the fact that this was
the first place in Rails we used `symbolize_names`, and that
`deep_symbolize_keys` had been used in the past.
This commit switches over to `deep_symbolize_keys` to allow `config_for`
to continue working for people using [safe_yaml].
[safe_yaml]: https://rubygems.org/gems/safe_yaml
be2fd61137