test(deps): update dependency org.mockito:mockito-core to v4 (#359)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [org.mockito:mockito-core](https://togithub.com/mockito/mockito) | `3.12.4` -> `4.0.0` | [![age](https://badges.renovateapi.com/packages/maven/org.mockito:mockito-core/4.0.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/org.mockito:mockito-core/4.0.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/org.mockito:mockito-core/4.0.0/compatibility-slim/3.12.4)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/org.mockito:mockito-core/4.0.0/confidence-slim/3.12.4)](https://docs.renovatebot.com/merge-confidence/) |

---

### Release Notes

<details>
<summary>mockito/mockito</summary>

### [`v4.0.0`](https://togithub.com/mockito/mockito/releases/v4.0.0)

[Compare Source](https://togithub.com/mockito/mockito/compare/v3.12.4...v4.0.0)

### Mockito 4: Removing deprecated APIs.

All of these APIs have been marked as deprecated and have been present
in Mockito for quite a while.

An overview of now-deleted classes/methods:

-   `org.mockito.Matchers` which was an alias for
    `org.mockito.ArgumentMatchers`
-   `org.mockito.ArgumentMatchers#{anyObject,anyVararg}` both which were
    aliases for `org.mockito.ArgumentMatchers#any`
-   `org.mockito.ArgumentMatchers#any*Of`, which were aliases for the same
    method name without the Of and the generic parameters (which were
    ignored)
-   `org.mockito.ArgumentMatchers#{is}{Not}Null(Class)` which took a class
    which was ignored. Aliases for the same methods without the parameter
-   `org.mockito.MockedStatic#verify` which had the parameter types reversed
-   `org.mockito.Mockito#verifyZeroInteractions` an alias of
    `verifyNoMoreInteractions`
-   `org.mockito.Mockito#debug` framework integration API that we later
    refactored
-   `org.mockito.configuration.AnnotationEngine` which was leaking internal
    APIs and instead users should use `org.mockito.plugins.AnnotationEngine`
-   `org.mockito.exceptions.verification.TooLittleActualInvocations` fixed
    the grammar from "Little" to "Few"
-   Numerous internal APIs that we never officially supported and can now
    remove
-   `org.mockito.plugins.InstantiatorProvider` which was leaking internal
    APIs and instead users should use InstantiatorProvider2 (we should
    probably rename back to remove the number in a future major release)
-   `org.mockito.runners` a package that hosted several old JUnit runners
    which were no longer supported. Users should instead use
    `org.mockito.junit.MockitoJUnitRunner` which is our official JUnit4
    runner.

</details>

---

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dialogflow-cx/snippets/pom.xml

@@ -55,7 +55,7 @@
     <dependency>
       <groupId>org.mockito</groupId>
       <artifactId>mockito-core</artifactId>
-      <version>3.12.4</version>
+      <version>4.0.0</version>
       <scope>test</scope>
     </dependency>
     <dependency>