Use Java 21 for tests

Use this link to re-run the recipe: https://app.moderne.io/builder/H0eeUs6x1?organizationId=ODQ2MGExMTUtNDg0My00N2EwLTgzMGMtNGE1NGExMTBmZDkw

Co-authored-by: Moderne <[email protected]>

Author: timtebeek

src/test/java/org/openrewrite/java/migrate/BouncyCastleTest.java

@@ -156,7 +156,7 @@ void runBouncyCastleArtifactUpgradeRecipe(
                   .singleElement()
                   .satisfies(rd -> {
                       assertThat(rd.getGroupId()).isEqualTo("org.bouncycastle");
-                      assertThat(rd.getArtifactId()).isEqualTo(String.format("%s-%s", baseArtifactId, expectedArtifactSuffix));
+                      assertThat(rd.getArtifactId()).isEqualTo("%s-%s".formatted(baseArtifactId, expectedArtifactSuffix));
                   }))
             )
           )