This cleans up our settings to not include any `version` fields, as we
have an actual settings migrator now.

This PR removes `language_models > anthropic > version`,
`language_models > openai > version` and `agent > version`.

We had migration paths in the code for a long time, so in practice
almost everyone should be using the latest version of these settings.


Release Notes:

- Remove `version` fields in settings for `agent`, `language_models >
anthropic`, `language_models > openai`. Your settings will automatically
be migrated. If you're running into issues with this open an issue
[here](https://github.com/zed-industries/zed/issues)