Update rustfmt.toml to 2021 edition

Bug: 210475268
Test: $ cd system/security; find . -name \*.rs -exec rustfmt --check {} \;
Change-Id: I49044e19a17d3806570d04c3215ccd3554ce7903
diff --git a/scripts/rustfmt.toml b/scripts/rustfmt.toml
index 617d425..cefaa42 100644
--- a/scripts/rustfmt.toml
+++ b/scripts/rustfmt.toml
@@ -1,5 +1,5 @@
 # Android Format Style
 
-edition = "2018"
+edition = "2021"
 use_small_heuristics = "Max"
 newline_style = "Unix"