[build] Fix and improve Java versions

gradle/libs.versions.toml

@@ -3,6 +3,8 @@ assertj = "3.26.3"
 asm = "9.7"
 autoservice = "1.1.1"
 acl = "1.3.5"
+java-main = "8"
+java-test = "21"
 jetbrains-annotations = "26.0.1"
 jspecify = "1.0.0"
 junit = "5.12.0"