refactor(@angular-devkit/core): deprecate stringToFileBuffer and file… #3543

Job	Run time
list	21s
workflow (modules-builder-tests)	1m 44s
workflow (devkit-core-tests-rerun)	1m 22s
workflow (build-unit-tests)	2m 32s
workflow (devkit-core-tests)	1m 18s
workflow (build-cli)	1m 32s
	8m 49s

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