Bump assertj-core from 3.20.2 to 3.21.0 in /core

Bumps [assertj-core](https://github.com/assertj/assertj-core) from 3.20.2 to 3.21.0.
- [Release notes](https://github.com/assertj/assertj-core/releases)
- [Commits](assertj/assertj@assertj-core-3.20.2...assertj-core-3.21.0)

---
updated-dependencies:
- dependency-name: org.assertj:assertj-core
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

core/build.gradle

@@ -100,7 +100,7 @@ dependencies {
     testImplementation files('testlib/repo/fakejar/fakejar/0/fakejar-0.jar')
 
     testImplementation 'org.rnorth.visible-assertions:visible-assertions:2.1.2'
-    testImplementation 'org.assertj:assertj-core:3.20.2'
+    testImplementation 'org.assertj:assertj-core:3.21.0'
     testImplementation project(':test-support')
 
     jarFileTestCompileOnly "org.projectlombok:lombok:${lombok.version}"