[infra] Bump spotless to a latest stable release (#7058)

We were using the beta release, which is not ideal since stable releases
are already out for the same major version.

Author: rlazo

gradle/libs.versions.toml

@@ -70,7 +70,7 @@ rxandroid = "2.0.2"
 rxjava = "2.1.14"
 serialization = "1.5.1"
 slf4jNop = "2.0.9"
-spotless = "7.0.0.BETA3"
+spotless = "7.0.4"
 testServices = "1.2.0"
 truth = "1.4.4"
 truthProtoExtension = "1.0"