THRIFT-5819: use latest rustc version for rustlib (#3085)

Client: rust

This upgrades the version of rust in the rust-toolchain file, docs and dockerfiles. Doing so requires a few changes to the source, mainly to fix or silence new warnings.

Submitted on behalf of a third-party: Jiayu Liu
Derived from the following PR: https://github.com/apache/thrift/pull/3045

Co-authored-by: Jiayu Liu <jiayu@hey.com>
diff --git a/test/rs/src/bin/test_client.rs b/test/rs/src/bin/test_client.rs
index fd3a185..801ccc4 100644
--- a/test/rs/src/bin/test_client.rs
+++ b/test/rs/src/bin/test_client.rs
@@ -252,10 +252,7 @@
     info!("testi64");
     // try!(verify_expected_result(thrift_test_client.test_i64(-8651829879438294565),
     // -8651829879438294565));
-    verify_expected_result(
-        thrift_test_client.test_i64(i64::min_value()),
-        i64::min_value(),
-    )?;
+    verify_expected_result(thrift_test_client.test_i64(i64::MIN), i64::MIN)?;
 
     info!("testDouble");
     verify_expected_result(