Do not suggest using Error in no_std before Rust 1.81 (#13834)

changelog: [`result_unit_err`]: do not suggest using `Error` in `no_std`
mode before Rust 1.81

Fix #9767

tests/ui/result_unit_error_no_std.rs

@@ -0,0 +1,26 @@
+#![feature(lang_items, start, libc)]
+#![no_std]
+#![warn(clippy::result_unit_err)]
+
+#[clippy::msrv = "1.80"]
+pub fn returns_unit_error_no_lint() -> Result<u32, ()> {
+    Err(())
+}
+
+#[clippy::msrv = "1.81"]
+pub fn returns_unit_error_lint() -> Result<u32, ()> {
+    Err(())
+}
+
+#[start]
+fn main(_argc: isize, _argv: *const *const u8) -> isize {
+    0
+}
+
+#[panic_handler]
+fn panic(_info: &core::panic::PanicInfo) -> ! {
+    loop {}
+}
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}

tests/ui/result_unit_error_no_std.stderr

@@ -0,0 +1,12 @@
+error: this returns a `Result<_, ()>`
+  --> tests/ui/result_unit_error_no_std.rs:11:1
+   |
+LL | pub fn returns_unit_error_lint() -> Result<u32, ()> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use a custom `Error` type instead
+   = note: `-D clippy::result-unit-err` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::result_unit_err)]`
+
+error: aborting due to 1 previous error
+