build: update Gin minimum Go version to 1.21 (#3960)

* build: update Gin minimum Go version to 1.21

- Update the minimum Go version requirement for Gin from `1.20` to `1.21` in both `debug.go` and `debug_test.go`
- Modify the warning message to reflect the new minimum Go version requirement in `debug.go`
- Adjust the test assertion to match the updated warning message in `debug_test.go`

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

* docs: refine project documentation and CI configurations

- Update supported Go versions for GitHub actions to `1.21` and `1.22`
- Specify the required Go version as `1.21` or above in README
- Change code block syntax to `sh` in installation and demo run instructions
- Remove empty lines in README sections
- Update project list formatting without changing the content

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

---------

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

debug.go

@@ -13,7 +13,7 @@ import (
 	"sync/atomic"
 )
 
-const ginSupportMinGoVer = 18
+const ginSupportMinGoVer = 21
 
 // IsDebugging returns true if the framework is running in debug mode.
 // Use SetMode(gin.ReleaseMode) to disable debug mode.
@@ -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 1.20+.
+		debugPrint(`[WARNING] Now Gin requires Go 1.21+.
 
 `)
 	}