fix(writers): drop version stamps from generated output

Generated files carried the RelSpec version and build date, so every
rebuild produced a diff even when the schema was unchanged.

- bun, gorm, drizzle: remove the GeneratedBy template field and line
- pgsql, mssql, sqlite: emit a constant "-- Generated by RelSpec"
- bun: replace the version header test with a version-free assertion

pkg/buildinfo is unchanged and still backs the CLI version.

pkg/writers/bun/templates.go

@@ -7,8 +7,7 @@ import (
 
 // modelTemplate defines the template for generating Bun models
 const modelTemplate = `// Code generated by relspecgo. DO NOT EDIT.
-{{if .GeneratedBy}}// {{.GeneratedBy}}
-{{end}}package {{.PackageName}}
+package {{.PackageName}}
 
 {{if .Imports -}}
 import (