Admob package is not working on Unity project

Question

I am a newbie at Unity. I am making a phone game on Unity. My games work on real device and I can compile it perfectly. But When I add the Admob sdk that named: GoogleMobileAds-v8.1.0 to my project. (Also, I do not know which version is the latest) It is not working. Also, I searched on too many forums. It seems like version issues. But I do not know how to fix it? My project is so simple. Just I want to show the ad on my phone. Here is the code. (I have used to plugin on that https://developers.google.com/admob/unity/quick-start?hl=en )

using GoogleMobileAds.Api;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
public class admob_test : MonoBehaviour
{
   
    public Text bilgi;

    BannerView _bannerView;
    void Start()
    {
        MobileAds.Initialize(initStatus => { });
        CreateBannerView();
    }
#if UNITY_ANDROID
    private string _adUnitId = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
#elif UNITY_IPHONE
  private string _adUnitId = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
#else
  private string _adUnitId = "unused";
#endif

    public void CreateBannerView()
    {
        Debug.Log("Creating banner view");

        // If we already have a banner, destroy the old one.
        if (_bannerView != null)
        {
            DestroyAd();
        }

        // Create a 320x50 banner at top of the screen
        _bannerView = new BannerView(_adUnitId, AdSize.Banner, AdPosition.Top);
    }

    public void LoadAd()
    {
        // create an instance of a banner view first.
        if (_bannerView == null)
        {
            CreateBannerView();
        }
        // create our request used to load the ad.
        AdRequest adRequest = new AdRequest.Builder().Build();
        adRequest.Keywords.Add("unity-admob-sample");

        // send the request to load the ad.
        Debug.Log("Loading banner ad.");
        _bannerView.LoadAd(adRequest);
    }

    public void DestroyAd()
    {
        if (_bannerView != null)
        {
            Debug.Log("Destroying banner ad.");
            _bannerView.Destroy();
            _bannerView = null;
        }
    }

    // Update is called once per frame
    public void BannerButton() {
        _bannerView.Destroy();
    }
    public void InterstatialButton() { }
    public void rewardedbutton() { }
}

And here are the warning messages:

You can see it in second image.APİ is deprecated. How can I fix it?