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).