Both Ubuntu 20.04 and Windows 2019 runners have been retired and are no
longer available.

Remove retired runners from CI

Starting Aug 07, 2025, Linux and Windows `arm64` standard hosted runners
in public repositories are generally available[^1]. This patch makes
these runners part of the test matrix, fixing necessary things along the
way to make it happen.

[^1]: https://github.blog/changelog/2025-08-07-arm64-hosted-runners-for-public-repositories-are-now-generally-available/

Add support of Linux/Windows `arm64` runners

macOS 13 runners are deprecated and will reach end-of-life soon. Replace
them with macOS 26 runners.

There are public partner runners available on GitHub, such as ARM
runners for Windows and Linux. Since we recently added their support, it
worth mentioning them.

Replace macOS 13 with macOS 26 in CI

PostgreSQL 18 is the most recent version of the PostgreSQL database,
released on September 25, 2025. It just makes sense to keep up with the
upstream versions if we can, hence this patch.

Add PostgreSQL 18

This patch bumps the following actions used in CI to the following
versions:

* `actions/checkout@v4` -> `@v5`
* `actions/setup-python@v6` -> `@v6`

Bump action versions used in CI

Zizmor is a static analysis tool for GitHub Actions. It detects a wide
range of common security flaws in typical CI/CD workflows. By
integrating Zizmor, this change ensures both the CI workflow and action
itself is automatically checked against these classes of
vulnerabilities.

Add Zizmor job and address all reported issues