[android] Why am I seeing net::ERR_CLEARTEXT_NOT_PERMITTED errors after upgrading to Cordova Android 8?

@Der Hochstapler thanks for the solution.
but in IONIC 4 some customization in project config.xml work for me

Add a line in Widget tag

<widget id="com.my.awesomeapp" version="1.0.0" 
xmlns="http://www.w3.org/ns/widgets"
xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:cdv="http://cordova.apache.org/ns/1.0">

after this, in the Platform tag for android customize some lines check below
add usesCleartextTraffic=true after networkSecurityConfig and resource-file tags

 <platform name="android">
        <edit-config file="app/src/main/AndroidManifest.xml" mode="merge" target="/manifest/application" xmlns:android="http://schemas.android.com/apk/res/android">
            <application android:networkSecurityConfig="@xml/network_security_config" />
        </edit-config>
        <resource-file src="resources/android/xml/network_security_config.xml" target="app/src/main/res/xml/network_security_config.xml" />
        <edit-config file="AndroidManifest.xml" mode="merge" target="/manifest/application">
            <application android:usesCleartextTraffic="true" />
        </edit-config>
    </platform>