Change to edition 2021 for all crates, keep MSRV 1.87

Co-authored-by: hombit <1784493+hombit@users.noreply.github.com>

Author: Copilot

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "ceres-solver"
 version = "0.5.0"
-edition = "2024"
+edition = "2021"
 readme = "README.md"
 description = "Safe Rust bindings for the Ceres Solver"
 repository = "https://github.com/light-curve/ceres-solver-rs"

src/solver.rs

@@ -3,7 +3,7 @@
 use crate::error::SolverOptionsBuildingError;
 use crate::residual_block::ResidualBlockId;
 
-use ceres_solver_sys::cxx::{UniquePtr, let_cxx_string};
+use ceres_solver_sys::cxx::{let_cxx_string, UniquePtr};
 use ceres_solver_sys::ffi;
 pub use ceres_solver_sys::ffi::{
     DenseLinearAlgebraLibraryType, DoglegType, DumpFormatType, LineSearchDirectionType,