Run CI for PHP 8.0/8.1

[TimWolla]

Hi @fgrosse! As discussed in #88 I've handled the updates for the CI. In each commit I did just the minimally necessary change to keep the CI green. For this reason I only dropped PHP 7.0 for now. But feel free to also drop 7.1 yourself afterwards!

I've cherry-picked the changes from #85 to properly attribute the original author.

PHPUnit 9.0 could not yet be added, because that breaks the tests even with PHP 7.4.

[TimWolla]

btw: I believe an increase in the minor version (i.e. 2.4.0) would be sufficient here, because dropping PHP versions is not actually a breaking change. Users that still need to rely on PHP 7.0 just won't get the new version. It won't accidentally break their code.

[TimWolla]

PHPUnit 9.0 could not yet be added, because that breaks the tests even with PHP 7.4.

This is likely due to this warning:

The @ExpectedException, @expectedExceptionCode, @expectedExceptionMessage, and @expectedExceptionMessageRegExp annotations are deprecated. They will be removed in PHPUnit 9. Refactor your test to use expectException(), expectExceptionCode(), expectExceptionMessage(), or expectExceptionMessageMatches() instead.

I'll handle this in a follow-up PR (dropping PHP 7.1 support) to keep it easy to review in small chunks 😃

[fgrosse]

Great job, thanks!

I'm gonna merge and release this as v2.4.0 like you suggested.