Pin `proptest` again to fix CI #2440

Job	Run time
benchmark	9m 38s
check_docs	1s
ext-test	1s
linting	2m 26s
check_release	2m 30s
fuzz	1s
build-tx-sync (ubuntu-latest, stable)	2m 44s
build-tx-sync (macos-latest, stable)	5m 19s
build-tx-sync (ubuntu-latest, 1.75.0)	5m 30s
build-tx-sync (macos-latest, 1.75.0)	7m 21s
build-tx-sync (ubuntu-latest, beta)	4m 42s
build (self-hosted, stable)	1s
build-tx-sync (macos-latest, beta)	6m 44s
rustfmt	14s
coverage	1s
build (macos-latest, 1.75.0)	1m 49s
build (self-hosted, beta)	1s
build (self-hosted, 1.75.0)	1s
build (macos-latest, stable)	5m 8s
build (windows-latest, stable)	9m 36s
	1h 3m 48s

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