Add support for PHP 8

[lepiaf]

PR is blocked by a dependency https://github.com/laminas/laminas-diagnostics/pull/15

composer update -n --prefer-dist --prefer-stable

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Updating dependencies

Your requirements could not be resolved to an installable set of packages.

  Problem 1

    - laminas/laminas-diagnostics[1.5.0, ..., 1.6.x-dev] require php ^5.6 || ^7.0 -> your php version (8.0.0) does not satisfy that requirement.

    - Root composer.json requires laminas/laminas-diagnostics ^1.5 -> satisfiable by laminas/laminas-diagnostics[1.5.0, 1.6.0, 1.6.x-dev].

We have to wait merge of PR https://github.com/laminas/laminas-diagnostics/pull/15 and a new version.

[kbond]

Great, thanks for working on this @lepiaf!

[kbond]

Hey @lepiaf, we just switched to GH actions so you'll have to rebase/adjust this PR (once laminas/laminas-diagnostics#15 is complete).

[kbond]

@lepiaf can you rebase?

[kbond]

Thanks @lepiaf!