[CI] Remove ubuntu-18.04 and add ubuntu-22.04

[ischoegl]

Changes proposed in this pull request

Update GitHub actions:

• Remove ubuntu-18.04 (faces brownouts in deprecation period and will be removed in December)
• Add ubuntu-22.04

If applicable, fill in the issue number this pull request is fixing

Closes #1370

If applicable, provide an example illustrating new features this pull request is introducing

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[bryanwweber]

Looks like the coverage build is broken? Is that related to the changes here?

[ischoegl]

Looks like the coverage build is broken? Is that related to the changes here?

I don't think so, but something about that CI run is odd. Let me retrigger.

[ischoegl]

Coverage ran through this time ... (which is a relief, as I hadn't touched that test)

[bryanwweber]

Thanks!