Support PHP 8

[jameswilddev]

Adds support for PHP 8. The following changes were necessary to get the builds running again:

• Composer 2 is installed by default, this isn't compatible with some of your dependencies so I've instructed it to roll back to Composer 1.
• Psalm 1.1.9 includes syntax which isn't compatible with older PHP versions, while 1.1.8 isn't able to install on PHP 8. I've split the difference by making it install different versions depending upon the PHP version.
• There was a use of a deprecated function which meant that the Psalm check was failing.
• Also updates https://github.com/DivineOmega/psr-18-guzzle-adapter to allow 8.0 support.

[coveralls]

Pull Request Test Coverage Report for Build 172

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.735%

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Totals
Change from base Build 166:	0.0%
Covered Lines:	85
Relevant Lines:	98

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💛 - Coveralls