Upgrade mockito-core to 3.4.6

Upgrades mockito version. Version 3.4.6 was chosen because 3.4.0
is the first one providing mocking of static methods that we possibly want to use.
Does not switch to mockito-inline yet.

metrics/micrometer/pom.xml

@@ -97,6 +97,7 @@
     <dependency>
       <groupId>org.mockito</groupId>
       <artifactId>mockito-core</artifactId>
+      <version>2.28.2</version>
       <scope>test</scope>
     </dependency>
     <dependency>

pom.xml

@@ -238,7 +238,7 @@
       <dependency>
         <groupId>org.mockito</groupId>
         <artifactId>mockito-core</artifactId>
-        <version>2.28.2</version>
+        <version>3.4.6</version>
       </dependency>
       <dependency>
         <groupId>io.reactivex.rxjava2</groupId>