docs: update examples to v4

[todgru]

Description

These changes update all markdown documentation to use the v4 versions of:

• actions/cache@v4
• actions/cache/save@v4
• actions/cache/restore@v4
• actions/checkout@v4

These changes do not update the .github/workflows. Those are addressed in: https://github.com/actions/cache/pull/1231

Motivation and Context

Up-to-date documentation improves developer happiness.

How Has This Been Tested?

Visual inspection of changes.

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation (add or update README or docs)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.