feature: allow usage of guzzlehttp/psr7 v2

[n00dl3]

replace deprecated function calls & bump min version of guzzlehttp/psr7 to 1.7+ or 2.0
related to #2099

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[n00dl3]

@googlebot I signed it!

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[n00dl3]

@googlebot I fixed it.

[n00dl3]

Oops, I didn't want to do that, I will remove this return type.

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[dwsupplee]

Thanks @n00dl3, this is great 👍.

[n00dl3]

Hello, I added a version check in the test, tests pass for 7.3+ I had difficulties compiling 7.2 so I couldn't test with this php version, but I think it should be okay.
Unfortunately, I was not able to run the tests on php 5.6 on my computer due to some php7+ syntax in other test files, maybe I am misunderstanding something.

[n00dl3]

This is the error I encountered, I guess tests are failing on php 5.6 since 1486680 .