Update JUnit dependencies in azure-identity

Currently azure-identity has a dependency on junit 4.13-beta-3. This should move in line with the other client libraries that are configured with the following xml:

<dependency>
  <groupId>org.junit.jupiter</groupId>
  <artifactId>junit-jupiter-api</artifactId>
  <version>5.4.2</version> <!-- {x-version-update;org.junit.jupiter:junit-jupiter-api;external_dependency} -->
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>org.junit.jupiter</groupId>
  <artifactId>junit-jupiter-engine</artifactId>
  <version>5.4.2</version> <!-- {x-version-update;org.junit.jupiter:junit-jupiter-engine;external_dependency} -->
  <scope>test</scope>
</dependency>

This should be copy/pasted in place of the existing JUnit 4 dependency. Additionally, it would be great if PowerMock could be removed, and mockito-junit-jupiter introduced instead. A good guide is available here.

[mclay]

Thanks @JonathanGiles please assign it to me and let me know the estimated due date.

[jianghaolu]

We will be able to move to mockito once we remove powermock after #7641.