3894 Bump min Android SDK to the version required at runtime (cla: yes, waiting for tree to go green, p: webview_flutter, p: google_maps_flutter, platform-android)

3900 Remove section from WebView readme (cla: yes, p: webview_flutter)

3926 [webview_flutter] Migrate maven repo from jcenter to mavenCentral (cla: yes, waiting for tree to go green, p: webview_flutter, platform-android)