fix: suggested fixes for FMT · VowpalWabbit/vowpal_wabbit@5aa3f9e

The following actions use a deprecated Node.js version and will be forced to run on node20: actions/checkout@v3, actions/cache@v3. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

'codecvt_utf8_utf16<char16_t>' is deprecated [-Wdeprecated-declarations]

'wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t>' is deprecated [-Wdeprecated-declarations]

'initialize' is deprecated: Replaced with new unique_ptr based overload. [-Wdeprecated-declarations]

'features' is deprecated: Moved into VW namespace. Will be removed in VW 10. [-Wdeprecated-declarations]

'features' is deprecated: Moved into VW namespace. Will be removed in VW 10. [-Wdeprecated-declarations]

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