chore(deps): update dependency minitest to v6

Author: renovate-bot

google-cloud-spanner/Gemfile

@@ -12,7 +12,7 @@ gem "google-cloud-spanner-v1", "~> 1.8"
 gem "google-style", "~> 1.31.0"
 gem "grpc", "~> 1.67"
 gem "grpc-tools", "~> 1.74", ">= 1.74.1"
-gem "minitest", "~> 5.25"
+gem "minitest", "~> 6.0"
 gem "minitest-autotest", "~> 1.0"
 gem "minitest-focus", "~> 1.4"
 gem "minitest-rg", "~> 5.3"