Fix TestContainers PostgreSQL module name for v2 compatibility

Update the module name from "postgresql" to "testcontainers-postgresql"
to align with TestContainers 2.x naming conventions where all database
container modules are prefixed with "testcontainers-".

This fixes the build failure:
"Could not find org.testcontainers:postgresql"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: nakamura-to

gradle/libs.versions.toml

@@ -40,7 +40,7 @@ junit-jupiter-api = { group = "org.junit.jupiter", name = "junit-jupiter-api" }
 junit-jupiter-engine = { group = "org.junit.jupiter", name = "junit-jupiter-engine" }
 junit-jupiter-params = { group = "org.junit.jupiter", name = "junit-jupiter-params" }
 junit-platform-launcher = { group = "org.junit.platform", name = "junit-platform-launcher" }
-testcontainers-postgresql = { group = "org.testcontainers", name = "postgresql" }
+testcontainers-postgresql = { group = "org.testcontainers", name = "testcontainers-postgresql" }
 
 [plugins]
 # Kotlin