chore(deps): update alchemist to v42.3.20 (#841)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

gradle/libs.versions.toml

@@ -1,5 +1,5 @@
 [versions]
-alchemist = "42.3.18"
+alchemist = "42.3.20"
 
 [libraries]
 alchemist = { module = "it.unibo.alchemist:alchemist", version.ref = "alchemist" }