chore(ci): Update actions/checkout action to v6

[kthy]

@coderabbitai ignore

This PR contains the following updates:

Package	Type	Update	Change
actions/checkout	action	major	v2 → v6

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Release Notes

actions/checkout (actions/checkout)

v6

Compare Source

• Fix tag handling: preserve annotations and explicit fetch-tags by @​ericsciple in #​2356

v5

Compare Source

• Port v6 cleanup to v5 by @​ericsciple in #​2301

v4

Compare Source

• Port v6 cleanup to v4 by @​ericsciple in #​2305

v3

Compare Source

• Fix: Mark test scripts with Bash'isms to be run via Bash
• Add option to fetch tags even if fetch-depth > 0

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Configuration

📅 Schedule: Branch creation - "before 9am" in timezone Europe/Berlin, Automerge - At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

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This PR has been generated by Renovate Bot.

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📝 Walkthrough

Walkthrough

Upgraded the GitHub Actions checkout step from v2 to v6 in the R-CMD-check workflow. This updates the action to a newer version with improved functionality and security patches.

Changes

Cohort / File(s)	Summary
GitHub Actions Workflow \.github/workflows/R-CMD-check\.yaml	Updated actions/checkout from v2 to v6 in the workflow step configuration

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

🐰 From version two to six we hop,
The checkout action takes a leap,
With newer features, security's tight,
Our CI/CD flows through the night!

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The pull request title clearly and specifically describes the main change: upgrading the actions/checkout action to v6 in the CI workflow.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

• 📝 Generate docstrings

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