Update Ubuntu runner from ubuntu-20.04 to ubuntu-latest in GitHub workflow

[Copilot]

Updates the deprecated ubuntu-20.04 runner to ubuntu-latest in the GitHub Actions workflow to ensure compatibility and security.

Changes Made

• Updated .github/workflows/tests.yaml line 16: os: ["ubuntu-20.04"] → os: ["ubuntu-latest"]

Benefits

• Future-proof: Always uses the most recent stable Ubuntu LTS version
• Security: Receives the latest security updates and patches
• Tooling: Includes updated tooling and dependencies
• Compliance: Avoids deprecation warnings as GitHub removes ubuntu-20.04 support

Before

matrix:
  os: ["ubuntu-20.04"]
  python_version: ["3.9", "3.10", "3.11", "3.12"]

After

matrix:
  os: ["ubuntu-latest"]
  python_version: ["3.9", "3.10", "3.11", "3.12"]

This is a minimal, non-breaking change that ensures the PostgreSQL playground tests continue to run on supported infrastructure.

Fixes #94.

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