build: upgrade golangci-lint to v2.13.1 and fix resulting lint issues - #122

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: DavidAnson/markdownlint-cli2-action@455b6612a7b7a80f28be9e019b70abdd11696e4e. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/