Refresh C++ client dependency and README

[dex-the-ai]

Summary

• bump couchbase_cxx_client from 1.0.5 to 1.3.1
• remove the redundant top-level find_package(fmt REQUIRED) because the newer dependency graph provides the needed fmt surface
• fix README drift around the required CMake version, env-helper script name, and test source filename

Verification

• cmake -S . -B build
• cmake --build build

Evidence

• tutorial-maintenance/runs/cxx-quickstart/2026-05-05T0145Z-subagent/verification.txt

Notes

• No qualifying Dex PR coverage existed for this repo when this follow-up started.

[dex-the-ai]

Owner ping for this maintenance PR: @shyam-cb

Requesting review from the manifest-listed owner(s) where GitHub does not already show a pending review request.

[gemini-code-assist]

Code Review

This pull request updates the couchbase_cxx_client dependency to version 1.3.1 and removes the explicit fmt package requirement in CMakeLists.txt. Additionally, the README.md has been updated to reflect the minimum CMake version requirement of 3.19, correct the environment setup script name to set_env_vars.sh, and fix a reference to the test source file. I have no feedback to provide.