ci: update Go version requirements and remove test files (#3957)

- Update the Go version requirements in `.github/workflows/gin.yml`
- Remove test files for Go versions 1.18 and 1.19
- Update the required Go version in `debug.go` and `debug_test.go`
- Rename and modify files related to Go version 1.19 and 1.20 in the `internal/bytesconv` directory

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

debug.go

@@ -78,7 +78,7 @@ func getMinVer(v string) (uint64, error) {
 
 func debugPrintWARNINGDefault() {
 	if v, e := getMinVer(runtime.Version()); e == nil && v < ginSupportMinGoVer {
-		debugPrint(`[WARNING] Now Gin requires Go 1.18+.
+		debugPrint(`[WARNING] Now Gin requires Go 1.20+.
 
 `)
 	}