refactor: remove calls to deprecated rand.Seed function (#5334)

The function has been deprecated since Go 1.20:
https://pkg.go.dev/math/rand#Seed.

Co-authored-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>

Author: andrzej-stencel

main.go

@@ -6,9 +6,7 @@ package main
 
 import (
 	"fmt"
-	"math/rand"
 	"os"
-	"time"
 
 	"github.com/elastic/elastic-agent/internal/pkg/agent/cmd"
 	"github.com/elastic/elastic-agent/pkg/core/process"
@@ -36,7 +34,6 @@ func main() {
 	}
 	defer pj.Close()
 
-	rand.Seed(time.Now().UnixNano())
 	command := cmd.NewCommand()
 	err = command.Execute()
 	if err != nil {

testing/integration/install_test.go

@@ -334,7 +334,6 @@ func TestRepeatedInstallUninstall(t *testing.T) {
 }
 
 func randStr(length int) string {
-	rand.Seed(time.Now().UnixNano())
 	var letters = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
 
 	runes := make([]rune, length)