[improve][test] Upgrade Mockito to 5.23.0

[lhotari]

Motivation

Pulsar relies on Mockito for unit testing across many modules. Since Mockito remains an actively used and integral test dependency, it should be kept up to date so that the project benefits from upstream bug fixes, JDK compatibility improvements, and reduced security/maintenance risk caused by drifting behind on widely-used libraries.

This PR bumps Mockito from 5.19.0 to 5.23.0, the current latest release.

Notable changes between 5.19.0 and 5.23.0 (per https://github.com/mockito/mockito/releases)

• 5.23.0 — Fix StackOverflowError with AbstractList after using mockSingleton; mark Mockito.when parameters @Nullable; (Android-only breaking change replacing the mockito-android mock-maker — does not affect Pulsar, which only consumes mockito-core).
• 5.22.0 — Avoid mocking of internal static utilities; fix static mocking of UUID.class corruption under JDK 25; add core API to enable Kotlin singleton mocking.
• 5.21.0 — Output matchers in potential mismatch messages (Fix semantics of skip output #2468); forbid mocking WeakReference with the inline mock maker (fixes StackOverflowError); support primitives in GenericArrayReturnType (fixes Android ClassNotFoundException for primitive arrays).
• 5.20.0 — Mock construction of generic types (Consumer often discards received messages #2401); JDK 21 sequenced collection support; ByteBuddy bump to 1.17.7.

The Mockito 5.23.0 transitive dependency on byte-buddy (1.17.7) already matches the version pinned in Pulsar's gradle/libs.versions.toml, and objenesis remains at 3.3 (unchanged from 5.19.0), so no other version updates are required.

Modifications

• Updated mockito version in gradle/libs.versions.toml from 5.19.0 to 5.23.0.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial dependency upgrade without functional code changes. It is covered by the existing test suite — verified locally with:

• ./gradlew assemble — BUILD SUCCESSFUL
• ./gradlew testClasses — BUILD SUCCESSFUL (all test sources compile against Mockito 5.23.0)
• ./gradlew spotlessCheck checkstyleMain checkstyleTest — BUILD SUCCESSFUL

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency) — Mockito test dependency upgraded from 5.19.0 to 5.23.0. Test scope only; no production code or runtime dependencies affected.
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment