chore(ci): bump markdown lint checkout action to v4

[bitcoin-tools]

No description provided.

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

OpenSSF Scorecard

Package	Version	Score	Details
actions/actions/checkout	2.*.*	🟢 7.5	Details Check Score Reason Code-Review 🟢 10 all changesets reviewed Maintained 🟢 10 12 commit(s) and 0 issue activity found in the last 90 days -- score normalized to 10 CII-Best-Practices ⚠️ 0 no effort to earn an OpenSSF best practices badge detected License 🟢 10 license file detected Branch-Protection ⚠️ -1 internal error: error during branchesHandler.setup: internal error: githubv4.Query: Resource not accessible by integration Binary-Artifacts 🟢 10 no binaries found in the repo Dangerous-Workflow 🟢 10 no dangerous workflow patterns detected Token-Permissions ⚠️ 0 detected GitHub workflow tokens with excessive permissions Signed-Releases ⚠️ -1 no releases found Fuzzing ⚠️ 0 project is not fuzzed Security-Policy 🟢 9 security policy file detected Packaging 🟢 10 packaging workflow detected SAST 🟢 10 SAST tool is run on all commits Pinned-Dependencies 🟢 4 dependency not pinned by hash detected -- score normalized to 4 Vulnerabilities 🟢 9 1 existing vulnerabilities detected

Scanned Manifest Files

.github/workflows/validation.yaml

• actions/checkout@2.*.*