Update checkout and setup-python to current majors in build workflow

[katsugtgz]

Updates actions/checkout to v7 and actions/setup-python to v7 in the build workflow.

Evidence:

• .github/workflows/ci.yml referenced actions/checkout@v3 and actions/setup-python@v3
• current stable releases are checkout v7.0.1 and setup-python v7.0.0

Validation:

• workflow YAML parses after the change
• the workflow does not use the pip-install input that setup-python v7 removed
• python matrix (3.8 to 3.12) and job steps are unchanged

Scope: .github/workflows/ci.yml only.