Mark old extensions file as deprecated #3635

Job	Run time
Test (7.2.1, ubuntu-latest, gcc)	2m 7s
Test (7.2.1, ubuntu-latest, clang)	1m 50s
Test (7.2.1, macos-latest, clang)	2m 58s
Test (7.2.1, windows-latest, msvc)	4m 17s
Test (7.5.0, ubuntu-latest, gcc)	2m 31s
Test (7.5.0, ubuntu-latest, clang)	1m 44s
Test (7.5.0, macos-latest, clang)	2m 30s
Test (7.5.0, windows-latest, msvc)	4m 30s
Test (8.0.1, ubuntu-latest, gcc)	2m 3s
Test (8.0.1, ubuntu-latest, clang)	1m 46s
Test (8.0.1, macos-latest, clang)	2m 9s
Test (8.0.1, windows-latest, msvc)	3m 21s
Test (latest, ubuntu-latest, gcc)	1m 46s
Test (latest, ubuntu-latest, clang)	1m 45s
Test (latest, macos-latest, clang)	2m 27s
Test (latest, windows-latest, msvc)	3m 55s
	41m 39s

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