Upgrade rust/crates/regex-syntax to 0.6.21

Test: make
Change-Id: I8c34d16c9a7049044418c71b091f19218c72526d
diff --git a/Cargo.toml.orig b/Cargo.toml.orig
index eb3ada9..7b56f32 100644
--- a/Cargo.toml.orig
+++ b/Cargo.toml.orig
@@ -1,6 +1,6 @@
 [package]
 name = "regex-syntax"
-version = "0.6.20"  #:version
+version = "0.6.21"  #:version
 authors = ["The Rust Project Developers"]
 license = "MIT/Apache-2.0"
 repository = "https://github.com/rust-lang/regex"