Remove ubuntu-16.04 from workflows

[mishina2228]

This PR removes ubuntu-16.04 from GitHub Actions workflows.

A workflow with ubuntu-16.04 is not running forever.
https://github.com/brianmario/mysql2/runs/6066060698?check_suite_focus=true

Requested labels: ubuntu-16.04
Job defined at: brianmario/mysql2/.github/workflows/build.yml@refs/pull/1254/merge
Waiting for a runner to pick up this job...

This is because GitHub Actions no longer supports Ubuntu 16.04.
https://github.blog/changelog/2021-04-29-github-actions-ubuntu-16-04-lts-virtual-environment-will-be-removed-on-september-20-2021/

• Currently available environments:
  https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources

But it seems that Ubuntu 16.04 LTS is supported until 2026.
So should we still run the CI on Ubuntu 16.04 like CentOS and Fedora listed in .github/workflows/container.yml?

[sodabrew]

Thanks, I agree there's no reason for 16.04 anymore. 22.04 is about to drop and going three LTS releases back is plenty (22.04, 20.04, 18.04)