[k6] Update k6 container image tag

[martincostello]

Closes #834

Updates the k6 image to the latest version.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• New integration
  • Docs are written
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

None.

[Copilot]

Pull Request Overview

Updates the k6 container image tag from version 0.57.0 to 1.2.3 to use the latest available version of the k6 load testing tool.

Key changes:

• Updated k6 container image tag to the latest version