chore: Replace deprecated io/ioutil functions with recommended alternatives (#9542)

# Which Problems Are Solved

- The `io/ioutil` package was deprecated in Go 1.16.  
  - Reference: https://go.dev/doc/go1.16#ioutil  

# How the Problems Are Solved

- Replaced deprecated functions with their recommended alternatives:  
  - `ioutil.ReadFile` → `os.ReadFile`  
  - `ioutil.ReadAll` → `io.ReadAll`  
  - `ioutil.NopCloser` → `io.NopCloser`

internal/notification/templates/template.go

@@ -3,7 +3,7 @@ package templates
 import (
 	"bytes"
 	"html/template"
-	"io/ioutil"
+	"io"
 	"net/http"
 )
 
@@ -51,7 +51,7 @@ func readFile(dir http.FileSystem, fileName string) (*template.Template, error)
 		return nil, err
 	}
 	defer f.Close()
-	content, err := ioutil.ReadAll(f)
+	content, err := io.ReadAll(f)
 	if err != nil {
 		return nil, err
 	}
@@ -68,7 +68,7 @@ func readFileFromDatabase(dir http.FileSystem, fileName string) (*template.Templ
 		return nil, err
 	}
 	defer f.Close()
-	content, err := ioutil.ReadAll(f)
+	content, err := io.ReadAll(f)
 	if err != nil {
 		return nil, err
 	}