add php 8 support

[datagutt]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Dependency update

• What is the current behavior? (You can also link to an open issue here)
  No support for PHP 8.x.

• What is the new behavior (if this is a feature change)?
  The package supports PHP 8.x. In addition to this, i updated the netflex/http, phpunit/phpunit and squizlabs/php_codesniffer packages to the latest available versions as of this PR.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No, this commit allows users of 7.4 to keep using the package. The difference is that 8.x is supported as well.

• Other information:
  The tests are passing. Might have missed something, so this should probably be tested further😄

[datagutt]

FYI, i have been running this in production for a while now (with Sentry-logging on), and haven't seen any errors or warnings so far.

[thomas-alrek]

Hello, and thank you for your PR. According to PHP's Type Declarations documentation, the mixed type is only available starting from PHP 8.0 forwards. I'm afraid this change will break support for PHP 7.4. We still use PHP 7.4 internally for some production systems, and use this package ourself. So we can't currently merge this before this issue is resolved.

[datagutt]

Ah, my mistake. PHP 7.x doesn't seem to support union types either, so there is no real way to fix this without breaking PHP 7.x support.