Upgrade base64 to 0.21 (#123)

Summary: Pull Request resolved: facebookresearch/Private-ID#123

Reviewed By: zertosh

Differential Revision: D61737689

fbshipit-source-id: dc98887dd6b346f11d8efe7ffa085311f6110c37

Author: capickett

shim/third-party/rust/Cargo.toml

@@ -38,7 +38,7 @@ async-scoped = { version = "0.8", features = ["use-tokio"] }
 async-trait = "0.1.24"
 atomic = "0.5.1"
 backtrace = "0.3.51"
-base64 = "0.13.0"
+base64 = "0.21.7"
 bincode = "1.3.3"
 bitflags = "2.4"
 blake3 = { version = "1.3.1", features = [ "default", "digest", "rayon", "std", "traits-preview" ] }