feat!: update minimum supported PHP version to 8.1

[tcarrio]

This PR

Updates the minimum supported version of the SDK to PHP 8.1
Introduces new PHP language features including readonly properties, enumerations,

• minimum supported version of PHP is now 8.1
• refactors pseudo-enums to true enums
• utilizes readonly properties where applicable
• utilizes array unpacking, e.g. [...$array] over array_merge($array)

Related Issues

Resolves #108

Follow-up Tasks

General TODOs before leaving DRAFT:

• Resolve refactor failures in tests and static analysis
• Update flagd provider to support breaking changes from major version release

How to test

Locally, you can run composer run test to execute the full test suite

Remotely, see GitHub CI runs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (82d58d2) 95.19% compared to head (21be2ee) 95.89%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #109      +/-   ##
============================================
+ Coverage     95.19%   95.89%   +0.69%     
+ Complexity      227      220       -7     
============================================
  Files            40       40              
  Lines           583      584       +1     
============================================
+ Hits            555      560       +5     
+ Misses           28       24       -4     

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