Problems with level 26 Android

Hello, I’m having this issue uploading my app to Google Play.

First, when I’m producing the jarsigner, I get this warning:

Warning: No -tsa or -tsacert is provided and this jar is not timestamped. Without a timestamp, users may not be able to validate this jar after the signer certificate's expiration date (2044-06-30) or after any future revocation date.

Ok, it’s just a warning, but when I upload the apk to Google Play, I get this warning that prevents from releasing the new version.

Your app currently targets API level 25 and must target at least API level 26 to ensure it is built on the latest     APIs optimized for security and performance.

From August 2018, new apps must target at least Android 8.0 (API level 26).
From November 2018, app updates must target Android 8.0 (API level 26).

Tip: Change your app's target API level. Learn how.

I’ve tried this solution, to change
<preference name="android-minSdkVersion" value="16" />
<preference name="android-minSdkVersion" value="19" />

I’m a little lost.