Bump org.mockito:mockito-core in /codegen in the gradle group

Bumps the gradle group in /codegen with 1 update: [org.mockito:mockito-core](https://github.com/mockito/mockito).


Updates `org.mockito:mockito-core` from 5.20.0 to 5.21.0
- [Release notes](https://github.com/mockito/mockito/releases)
- [Commits](mockito/mockito@v5.20.0...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-group: gradle
...

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

Author: dependabot[bot]

codegen/gradle/libs.versions.toml

@@ -1,7 +1,7 @@
 [versions]
 junit5 = "6.0.2"
 smithy = "1.65.0"
-mockito = "5.20.0"
+mockito = "5.21.0"
 test-logger-plugin = "4.0.0"
 spotbugs = "6.0.22"
 spotless = "8.1.0"