Replace fmt.Sprintf with hex.EncodeToString (#21960)

`hex.EncodeToString` has better performance than `fmt.Sprintf("%x",
[]byte)`, we should use it as much as possible.

I'm not an extreme fan of performance, so I think there are some
exceptions:

- `fmt.Sprintf("%x", func(...)[N]byte())`
- We can't slice the function return value directly, and it's not worth
adding lines.
    ```diff
    func A()[20]byte { ... }
    - a := fmt.Sprintf("%x", A())
    - a := hex.EncodeToString(A()[:]) // invalid
    + tmp := A()
    + a := hex.EncodeToString(tmp[:])
    ```
- `fmt.Sprintf("%X", []byte)`
- `strings.ToUpper(hex.EncodeToString(bytes))` has even worse
performance.

routers/api/packages/maven/maven.go

@@ -8,9 +8,9 @@ import (
 	"crypto/sha1"
 	"crypto/sha256"
 	"crypto/sha512"
+	"encoding/hex"
 	"encoding/xml"
 	"errors"
-	"fmt"
 	"io"
 	"net/http"
 	"path/filepath"
@@ -128,7 +128,7 @@ func serveMavenMetadata(ctx *context.Context, params parameters) {
 			tmp := sha512.Sum512(xmlMetadataWithHeader)
 			hash = tmp[:]
 		}
-		ctx.PlainText(http.StatusOK, fmt.Sprintf("%x", hash))
+		ctx.PlainText(http.StatusOK, hex.EncodeToString(hash))
 		return
 	}