go.mod: fix go version format (#1682)

methane · web-flow · commit b84ac5af9c77 · 2025-03-14T22:05:00.000+09:00
As of Go 1.21, toolchain versions must use the 1.N.P syntax. https://go.dev/doc/toolchain#version
diff --git a/go.mod b/go.mod
@@ -1,5 +1,5 @@
 module github.com/go-sql-driver/mysql
 
-go 1.21
+go 1.21.0
 
 require filippo.io/edwards25519 v1.1.0
