Prevent use with rust 1.82+ (#1371)

### What
Prevent use with rust 1.82+. And run CI builds with 1.81.

### Why
stellar/rs-soroban-env#1469

Discussion:
https://stellarfoundation.slack.com/archives/C030Z9EHVQE/p1729217606684609
(cherry picked from commit 9e674f2)

rust-toolchain.toml

@@ -1,4 +1,4 @@
 [toolchain]
-channel = "stable"
+channel = "1.81"
 targets = ["wasm32-unknown-unknown"]
 components = ["rustc", "cargo", "rustfmt", "clippy", "rust-src"]

soroban-sdk/Cargo.toml

@@ -15,6 +15,9 @@ exclude = ["test_snapshots/", "src/tests/"]
 [lib]
 doctest = false
 
+[build-dependencies]
+rustc_version = "0.4.1"
+
 [dependencies]
 soroban-sdk-macros = { workspace = true }
 bytes-lit = "0.0.5"

soroban-sdk/build.rs

@@ -0,0 +1,8 @@
+pub fn main() {
+    #[cfg(all(target_family = "wasm", target_os = "unknown"))]
+    if let Ok(version) = rustc_version::version() {
+        if version.major == 1 && version.minor >= 82 {
+            panic!("Rust compiler 1.82+ with target 'wasm32-unknown-unknown' is unsupported by the Soroban Environment, because the 'wasm32-unknown-unknown' target in Rust 1.82+ has features enabled that are not yet supported and not easily disabled: reference-types, multi-value. Use Rust 1.81 to build for the 'wasm32-unknown-unknown' target.");
+        }
+    }
+}

soroban-sdk/src/lib.rs

@@ -47,12 +47,6 @@
 #[cfg(all(target_family = "wasm", feature = "testutils"))]
 compile_error!("'testutils' feature is not supported on 'wasm' target");
 
-#[cfg(all(target_family = "wasm", target_feature = "reference-types"))]
-compile_error!("Compiler configuration is unsupported by Soroban Environment, because a Wasm target-feature is enabled that is not yet supported: reference-types. Use Rust 1.81 or older to get a compatible default configuration.");
-
-#[cfg(all(target_family = "wasm", target_feature = "multivalue"))]
-compile_error!("Compiler configuration is unsupported by Soroban Environment, because a Wasm target-feature is enabled that is not yet supported: multivalue. Use Rust 1.81 or older to get a compatible default configuration.");
-
 // When used in a no_std contract, provide a panic handler as one is required.
 #[cfg(all(not(feature = "alloc"), target_family = "wasm"))]
 #[panic_handler]