[MAINT] Bump outdated `actions/*` references for version consistency

[rhoerr]

Background

While auditing the workflows and composite actions in this repo, I noticed wide major-version drift in the first-party actions/* dependencies. Most of these have new majors available with security fixes, smaller bundle sizes, and updated Node runners.

Findings

Action	Versions currently in use	Latest
actions/cache	v3, v4	v4
actions/cache/restore	v4	v4
actions/cache/save	v3	v4
actions/checkout	v3, v4, v6	v6
actions/upload-artifact	v4	v4

Files where the older versions appear:

• cache-magento/action.yml — actions/cache@v3
• nx-integration-tests-setup/action.yml — actions/cache@v3, actions/cache/save@v3
• unit-test/action.yml — actions/cache@v3
• supported-services-matrix-calculator/action.yml — actions/checkout@v3
• setup-di-compile/action.yml — actions/checkout@v3
• .github/workflows/_internal-get-composer-version.yaml — actions/checkout@v4
• .github/workflows/_internal-get-magento-version.yaml — actions/checkout@v4
• .github/workflows/_internal-integration.yaml — actions/checkout@v4
• .github/workflows/_internal-semver-compare.yaml — actions/checkout@v4
• .github/workflows/_internal-setup-magento.yaml — actions/checkout@v4
• .github/workflows/_internal-unit.yaml — actions/checkout@v4
• .github/workflows/_internal_test_actions.yaml — actions/checkout@v4
• .github/workflows/full-integration-tests.yaml — actions/checkout@v4
• .github/workflows/integration.yaml — actions/checkout@v4, actions/cache@v4, actions/upload-artifact@v4
• (_internal-coding-standard.yaml and sansec-ecomscan.yml already use actions/checkout@v6)

Proposed change

Standardize on the latest major across the repo:

• actions/cache → v4 everywhere (including the cache/save and cache/restore sub-paths)
• actions/checkout → v6 everywhere
• actions/upload-artifact → already on v4

Why

• Reduces the surface area of the GitHub Actions allowlist consumers need to maintain (one major per action instead of three).
• Pulls in updated Node runtimes and any security fixes that have shipped in newer majors.
• Makes future audits (like this one) trivially mechanical.

Out of scope

Third-party actions (shivammathur/setup-php, dorny/paths-filter, tj-actions/changed-files, etc.) — those will be handled in a separate issue covering the Node 20 → Node 24 migration.