Update composer.json

[oakleaf]

Description

Update composer.json to support Php 7.3+ and Php 8.0+, together with Guzzle 7.0+

Checklist

• Code compiles correctly
• Code conforms PSR-2. Recommending use PHP CS Fixer with next rules: --rules=@PSR2,blank_line_before_return,no_unused_imports
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README, if necessary
• Added myself / the copyright holder to the authors in composer.json file