Update dependency org.junit.jupiter:junit-jupiter-api to v6

Author: renovate[bot]

checker/build.gradle

@@ -83,7 +83,7 @@ dependencies {
     // For the Resource Leak Checker's support for IOUtils.
     testImplementation 'commons-io:commons-io:2.21.0'
     // For the Nullness Checker test of junit-assertions.astub in JUnitNull.java
-    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.14.2'
+    testImplementation 'org.junit.jupiter:junit-jupiter-api:6.0.3'
     testImplementation 'org.apiguardian:apiguardian-api:1.1.2'
     // For tests that use JSpecify annotations
     testImplementation 'org.jspecify:jspecify:1.0.0'