[SPARK-53770] Upgrade mockito to 5.20.0

### What changes were proposed in this pull request?

This PR aims to upgrade `mockito` to 5.20.0 for Apache Spark K8s Operator `v0.6`.

### Why are the changes needed?

To bring the latest features and bug fixes.
- https://github.com/mockito/mockito/releases/tag/v5.20.0
  - mockito/mockito#3729
  - mockito/mockito#3709
- https://github.com/mockito/mockito/releases/tag/v5.19.0
  - mockito/mockito#3708
  - mockito/mockito#3659

### Does this PR introduce _any_ user-facing change?

No behavior change.

### How was this patch tested?

Pass the CIs.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #358 from dongjoon-hyun/SPARK-53770.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

gradle/libs.versions.toml

@@ -25,7 +25,7 @@ log4j = "2.24.3"
 # Test
 junit = "5.13.4"
 jacoco = "0.8.13"
-mockito = "5.18.0"
+mockito = "5.20.0"
 
 # Build Analysis
 checkstyle = "10.23.1"