Rename nthPrimeApprox.cpp to RiemannR.cpp #70

Job	Run time
macos_clang (macos-latest, Debug)	2m 58s
macos_clang (macos-latest, Release)	1m 15s
linux_gcc_sanitizers	2m 21s
linux_clang (ubuntu-latest, Debug)	3m 35s
linux_gcc (ubuntu-latest, Debug)	4m 43s
linux_clang (ubuntu-latest, Release)	1m 10s
linux_gcc (ubuntu-latest, Release)	2m 7s
linux_gcc_valgrind	1m 11s
windows_mingw64 (windows-latest, Debug)	5m 22s
windows_msvc2022	1m 40s
windows_mingw64 (windows-latest, Release)	3m 0s
	29m 22s

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