bump Rust to 1.85

.gitlab-ci.yml

 # WARNING: update `rust-toolchain.toml` as well
-image: "rust:1.78"
+image: "rust:1.85"
 
 stages:
   - fmt

rust-toolchain.toml

 [toolchain]
 profile = "minimal"
 # WARNING: update `.gitlab-ci.yml` as well
-channel = "1.78"
+channel = "1.85"
 components = ["rustfmt", "clippy", "rust-analyzer"]