Fixes deprecations errors on php 8.1

[hevertonfreitas]

Since the library is meant to support php 7.1, some attributes were added to methods in a few classes (CurrentWeatherGroup, WeatherForecast and Forecast), fixing deprecations errors being thrown on php 8.1

[jamieburchell]

@cmfcmf 👋🏻 Please can we get this merged to fix PHP 8.1 deprecation notices?

[codecov]

Codecov Report

Merging #181 (ad578ec) into main (5a620de) will increase coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head ad578ec differs from pull request most recent head ac8bcf8. Consider uploading reports for the commit ac8bcf8 to get more accurate results

@@             Coverage Diff              @@
##               main     #181      +/-   ##
============================================
+ Coverage     90.28%   90.35%   +0.06%     
- Complexity      179      180       +1     
============================================
  Files            18       18              
  Lines           422      425       +3     
============================================
+ Hits            381      384       +3     
  Misses           41       41              

Impacted Files	Coverage Δ
Cmfcmf/OpenWeatherMap/CurrentWeatherGroup.php	100.00% <ø> (ø)
Cmfcmf/OpenWeatherMap/Util/Unit.php	100.00% <ø> (ø)
Cmfcmf/OpenWeatherMap/WeatherForecast.php	92.30% <ø> (ø)
Cmfcmf/OpenWeatherMap/Forecast.php	96.87% <100.00%> (+0.32%)	⬆️

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[cmfcmf]

Thanks a lot, and sorry for the long delay!
I also modified CI to fail on deprecation notices, so issues like this hopefully don't come up again in the future (5a620de).