Declare MSRV in Cargo.toml

Let's make our MSRV explicit.

Author: zeenix

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+- Make MSRV of 1.87.0 explicit.
+
 ## [v0.9.1] - 2025-08-19
 
 ### Added

Cargo.toml

@@ -8,6 +8,7 @@ categories = ["data-structures", "no-std"]
 description = "`static` friendly data structures that don't require dynamic memory allocation"
 documentation = "https://docs.rs/heapless"
 edition = "2021"
+rust-version = "1.87"
 keywords = ["static", "no-heap"]
 license = "MIT OR Apache-2.0"
 name = "heapless"