Revert "Restructure .gitignore gem handling" #32226

Job	Run time
syntax	42s
cask audit	2m 59s
formula audit	5m 37s
tap syntax	6m 16s
test default formula (Ubuntu 22.04)	1m 52s
tests (online)	5m 25s
test default formula (Ubuntu 20.04)	3m 14s
update-test (Ubuntu)	15s
test default formula (macOS 13 x86_64)	3m 47s
tests (generic OS)	3m 13s
vendored gems	42s
update-test (macOS)	24s
tests (Ubuntu 24.04)	3m 29s
test default formula (macOS 15 arm64)	1m 23s
tests (Ubuntu 22.04)	3m 39s
tests (Ubuntu 20.04)	3m 25s
tests (macOS 13 x86_64)	10m 13s
tests (macOS 15 arm64)	5m 22s
	1h 1m 57s

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