Revert 'mockito-core' to 3.12.4

With 'mockito-core' 4.0.0 an bug in combination with powermock is
introduced [1]. So the version is reverted to 3.12.4, the last version
before 4.0.0.

See: [1] https://github.com/powermock/powermock/issues/1109

Signed-off-by: Tim Krüger <t@timkrueger.me>

app/build.gradle

@@ -292,7 +292,7 @@ dependencies {
     implementation 'androidx.core:core-ktx:1.6.0'
 
     testImplementation 'junit:junit:4.13.2'
-    testImplementation 'org.mockito:mockito-core:4.0.0'
+    testImplementation 'org.mockito:mockito-core:3.12.4'
     testImplementation "org.powermock:powermock-core:${powermockVersion}"
     testImplementation "org.powermock:powermock-module-junit4:${powermockVersion}"
     testImplementation "org.powermock:powermock-api-mockito2:${powermockVersion}"