Bump com.diffplug.spotless from 6.16.0 to 6.18.0 #3112

Tests

• ❌ key.core.proof_references:test (87 succeeded, 2 failed)
• ❌ key.core.symbolic_execution:test (286 succeeded, 75 failed, 39 skipped)
• ✅ key.core.testgen:test (13 succeeded)
• ✅ key.core:test (2944 succeeded, 102 skipped)
• ✅ key.core:testProveRules (196 succeeded)
• ✅ key.core:testRunAllFunProofs (34 succeeded)
• ✅ key.core:testRunAllInfProofs (29 succeeded)
• ✅ key.removegenerics:test (45 succeeded)
• ✅ key.ui:test (5 succeeded)
• ✅ key.util:test (132 succeeded)
• ✅ keyext.exploration:test (3 succeeded)
• ✅ keyext.slicing:test (12 succeeded)
• ✅ recoder:test (14 succeeded, 15 skipped)