Remove deprecated `bearerToken` option

[toddbaert]

The authHeader option can do the same thing as the bearerToken option and is more flexible.

Please remove this option from viper, and all references:

flagd/docs/reference/sync-configuration.md

Line 55 in c07ffba

| bearerToken | optional `string` | (Deprecated) Used for http sync; token gets appended to `Authorization` header with [bearer schema](https://www.rfc-editor.org/rfc/rfc6750#section-2.1). Cannot be used with `authHeader` |

[m-olko]

Hi, I want to start contributing to flagd. After discussion with @tangenti, this looks like a good first issue. Can I start working on it if nobody has taken it?