Prepare the 1.21.0 stable release

This prepares CI to produce the 1.21.0 stable release, turned out we didn't need
many more changes other than this!

Author: alexcrichton

src/ci/run.sh

@@ -43,7 +43,7 @@ fi
 # FIXME: need a scheme for changing this `nightly` value to `beta` and `stable`
 #        either automatically or manually.
 if [ "$DEPLOY$DEPLOY_ALT" != "" ]; then
-  RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --release-channel=beta"
+  RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --release-channel=stable"
   RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-llvm-static-stdcpp"
 
   if [ "$NO_LLVM_ASSERTIONS" = "1" ]; then