Replace `MAPBOX_ETERNAL_CONSTEXPR` with `constexpr`

[louwers]

I don't think we need to support gcc 4.9 anymore.

[github-actions[bot]]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  [ = ]       0  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-2449-compared-to-main.txt

[github-actions[bot]]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  -0.0% -1.94Ki  -0.0% -1.44Ki    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2449-compared-to-main.txt

Compared to d38709084a9865fe0bb8300aec70ebf8243b3d43 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +21% +24.1Mi  +409% +24.4Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2449-compared-to-legacy.txt

[github-actions[bot]]

Benchmark Results ⚡

Benchmark                                                     Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------------------
OVERALL_GEOMEAN                                            +0.0006         +0.0008             0             0             0             0

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/benchmark-results/pr-2449-compared-to-main.txt