Replace Anthropic library (anthropic-rs -> anthropic) and switch default recommended model (claude-3-5-sonnet-20240620 -> claude-3-7-sonnet-20250219)

Fixes https://github.com/etkecc/baibot/issues/22

Ultimate related to `anthropic-rs` hardcoding models as enumeration variants in the code
and not updating them. See:
- https://github.com/roushou/mesh/issues/1
- https://github.com/roushou/mesh/pull/2

https://github.com/cortesi/misanthropy was also considered as an
alternative, but it did not allow configuring the base API URL like our
old Anthropic library (`anthropic-rs`) and like our new choice (`anthropic`).
We'd rather not lose support for this, so we're going with the `anthropic` library.

Cargo.toml

@@ -15,7 +15,7 @@ name = "baibot"
 path = "src/lib.rs"
 
 [dependencies]
-anthropic-rs = "0.1.*"
+anthropic = "=0.0.8"
 anyhow = "1.0.*"
 async-openai = "0.27.*"
 base64 = "0.22.*"