feat: update guzzle to version 7

[c0nst4ntin]

As raised in Issue Upgrade to Guzzle 7.x #170 Guzzle 7 is currently the most requested feature by the community.

This Pull-Request combines and improves changes from:

• Support GuzzleHTTP 7.x #184
• Guzzle ^7.0 & PHPUnit ^9.0 #171
• Updates Guzzle and PHP minimum version (7.1 is EOL) #168

During local testing it was possible to support both Guzzle six and seven.

[c0nst4ntin]

@gsteel Do you see any problem with releasing this as Version 5.3.0?

Should I drop ^6.3 from the composer.json and update the lock file again? Or keep it as it is?

[gsteel]

According to Travis, Guzzle was installed at ^7.0 - if you can't test ^6 in CI, how do we know it works with this patch? I'd just drop ^6, release a minor and be done with it. You will have to update the lockfile, yes.

[c0nst4ntin]

According to Travis, Guzzle was installed at ^7.0 - if you can't test ^6 in CI, how do we know it works with this patch? I'd just drop ^6, release a minor and be done with it. You will have to update the lockfile, yes.

Dropped support for Guzzle 6 in 93eaf2e

[c0nst4ntin]

I added your suggestion. Should I merge and release now?

[gsteel]

I added your suggestion. Should I merge and release now?

You're the boss 😉