go.mod: fix go version format

methane · methane · commit 941dc48693ef · 2025-03-10T11:38:50.000+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
