Use the development credentials provider in development by default (#25273)

This PR changes the default credentials provider used in developments
builds of Zed to the development credentials provider.

Previously this required setting `ZED_DEVELOPMENT_AUTH=1` in order to
opt-in to the development credentials provider.

This led to confusion for new Zed employees who did not know that this
environment variable existed.

If you do need to interact with the system keychain for some reason, you
can run Zed with:

```
ZED_DEVELOPMENT_USE_KEYCHAIN=1
```

`ZED_DEVELOPMENT_AUTH` is dead. Long live Zed development auth!

Release Notes:

- N/A