Update dependency com.google.ads.mediation:facebook to v6.20.0.0

Author: renovate[bot]

main/build.gradle

@@ -241,7 +241,7 @@ dependencies {
     // Admob
     implementation 'com.google.android.gms:play-services-ads:23.6.0'
     // Facebook Ad network
-    implementation 'com.google.ads.mediation:facebook:6.19.0.0'
+    implementation 'com.google.ads.mediation:facebook:6.20.0.0'
     // Adcolony Ad network
     implementation 'com.google.ads.mediation:adcolony:4.8.0.2'
     // To fix build failed with issue