Remove `XGBoosterGetModelRaw`. #8024

Job	Run time
Run R linters on OS ubuntu-latest, R release, Compiler , Build	11m 42s
Test R package on Debian	28m 42s
Test R on OS windows-latest, R release, Compiler mingw, Build autotools	11m 27s
Test R on OS windows-latest, R 4.2.0, Compiler msvc, Build cmake	14m 26s
	1h 6m 17s

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