Migrate no_std_test to edition 2021

Previously we had dependency problems that were resolved by resolver v2.
We want to activate it just in case it happens again but even better,
bump the edition.  This was probably forgotten when other crates were
migrated.

Author: Kixunil

no_std_test/Cargo.toml

@@ -2,6 +2,7 @@
 name = "no_std_test"
 version = "0.1.0"
 authors = ["Elichai Turkel <elichai.turkel@gmail.com>"]
+edition = "2021"
 
 [features]
 alloc = ["secp256k1/alloc", "wee_alloc"]