Bump mockitoVersion from 5.19.0 to 5.21.0 in /Plan

Bumps `mockitoVersion` from 5.19.0 to 5.21.0.

Updates `org.mockito:mockito-core` from 5.19.0 to 5.21.0
- [Release notes](https://github.com/mockito/mockito/releases)
- [Commits](mockito/mockito@v5.19.0...v5.21.0)

Updates `org.mockito:mockito-junit-jupiter` from 5.19.0 to 5.21.0
- [Release notes](https://github.com/mockito/mockito/releases)
- [Commits](mockito/mockito@v5.19.0...v5.21.0)

---
updated-dependencies:
- dependency-name: org.mockito:mockito-core
  dependency-version: 5.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.mockito:mockito-junit-jupiter
  dependency-version: 5.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Plan/build.gradle

@@ -91,7 +91,7 @@ subprojects {
         nkPlaceholderapiVersion = "1.4-SNAPSHOT"
 
         junitVersion = "6.0.1"
-        mockitoVersion = "5.19.0"
+        mockitoVersion = "5.21.0"
         seleniumVersion = "4.35.0"
         testContainersVersion = "1.21.3"
         awaitilityVersion = "4.3.0"