Back out "Upgrade Rust toolchain: 1.86 -> 1.89"

This backs out commit 3285f19d09.

.gitlab-ci.yml

@@ -1,5 +1,5 @@
 build, test and lint:
-    image: "rust:1.89-trixie"
+    image: "rust:1.86-bookworm"
     interruptible: true
     script:
         - apt update -qy

Cargo.toml

@@ -5,7 +5,7 @@ authors = ["Vincent Breitmoser <look@my.amazin.horse>", "Kai Michaelis <kai@sequ
 build = "build.rs"
 default-run = "hagrid"
 edition = "2024"
-rust-version = "1.89"
+rust-version = "1.86"
 resolver = "3"
 
 [workspace]

clippy.toml

@@ -1,3 +1,3 @@
-msrv = "1.89"
+msrv = "1.86"
 
 too-many-arguments-threshold = 10

rust-toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "1.89"
+channel = "1.86"
 components = ["rustfmt", "clippy"]