bump const_sv2 major to 3.0.0..

since v37, cargo semver-checks enforces that removal of features are breaking changes

therefore we need to update serde_v2 as a follow up to stratum-mining#1230

Author: plebhash

protocols/v2/const-sv2/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "const_sv2"
-version = "2.1.0"
+version = "3.0.0"
 authors = ["The Stratum V2 Developers"]
 edition = "2018"
 readme = "README.md"