modernize the codebase

Run the modernize linter across the codebase.
Change generated by running:

```
go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...
```

Ref:
https://pkg.go.dev/golang.org/x/tools/gopls/internal/analysis/modernize

internal/services/postService.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"math/rand"
 	"net/http"
+	"slices"
 	"strings"
 
 	"gorm.io/gorm"
@@ -163,10 +164,5 @@ func (ps *PostService) GetMedia() domain.MediaAttachment {
 }
 
 func arrayContains(arr []string, str string) bool {
-	for _, a := range arr {
-		if a == str {
-			return true
-		}
-	}
-	return false
-}
+	return slices.Contains(arr, str)
+}