Move regex-capi to 2021 & inline format-args #1971

Job	Run time
testfull-regex-automata	14m 51s
testfull-regex	7m 0s
msrv	4m 24s
miri-regex-automata	10m 24s
rustfmt	23s
test (stable, ubuntu-latest, stable)	7m 25s
testfull-regex-capi	50s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	5m 22s
testfull-regex-syntax	3m 31s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	11m 59s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	14m 24s
test (beta, ubuntu-latest, beta)	6m 49s
test (nightly, ubuntu-latest, nightly)	7m 19s
test (macos, macos-latest, stable)	4m 26s
test (win-msvc, windows-latest, stable)	10m 35s
test (win-gnu, windows-latest, stable-x86_64-gnu)	6m 2s
	1h 55m 44s

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