Remove `EncodingConfigAdapter` and legacy `EncodingConfig`s

[pablosichert]

Once #9459 is completed, we can follow up to remove the EncodingConfigAdapter and legacy EncodingConfigs.

These are currently needed to integrate the new Encoder with sinks without breaking backwards-compatibility. However at the expense of bad error messages when the config is malformed/invalid:

• the serde deny_unkown_fields attribute doesn't work in combination with flatten and untagged enums (deny_unknown_fields incorrectly fails with flattened untagged enum serde-rs/serde#1600), therefore additional "junk" is not rejected from configs
• untagged enums in serde currently don't report detailed errors: Collect errors when deserializing untagged enums serde-rs/serde#1544