Update GitHub checkout action

[hpoettker]

Thanks for merging #367! The build on dev is now green. 💚 🚀

To keep it that way, I think the following warning from the current builds is noteworthy:

Node.js 20 actions are deprecated. The following actions are running on Node.js 20 and may not work as expected: actions/checkout@v2. Actions will be forced to run with Node.js 24 by default starting June 2nd, 2026. Node.js 20 will be removed from the runner on September 16th, 2026. Please check if updated versions of these actions are available that support Node.js 24. To opt into Node.js 24 now, set the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true environment variable on the runner or in your workflow file. Once Node.js 24 becomes the default, you can temporarily opt out by setting ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

This PR upgrades the action from v2 to the current latest major version v6, which successfully removes the warning.

For unrelated reasons, I've amended the .gitignore by some entries:

• configure~ is created by the autotools on autoreconf-ing an existing configure script
• m4 is being created due to AC_CONFIG_MACRO_DIRS from Drop gettext as no longer needed #342
• .vs is created when using Visual Studio (without the Code)

[evanmiller]

Thanks!