quinn: Make SendStream::poll_stopped private #5239

Job	Run time
audit	25s
msrv	1m 12s
lint	1m 3s
features (ubuntu-latest)	1h 20m 30s
features (macos-latest)	26m 53s
test (ubuntu-latest, stable)	1m 28s
features (windows-latest)	48m 49s
test-android (x86_64-linux-android, x86_64, 26)	2m 58s
test-aws-lc-rs	2m 1s
test (ubuntu-latest, beta)	1m 18s
test-android (x86_64-linux-android, x86_64, 25)	2m 42s
test (macos-latest, stable)	1m 10s
test-android (i686-linux-android, x86, 26)	4m 16s
test (windows-latest, stable)	2m 54s
test on freebsd	4m 23s
test on illumos	7m 22s
test-android (i686-linux-android, x86, 25)	2m 24s
test on netbsd	4m 59s
test wasm32-unknown-unknown	48s
test on solaris	14m 18s
	3h 31m 53s

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