Bump librdkafka to 2.3.0. #532

Job	Run time
lint	7m 24s
check-minimal-versions	3m 58s
test (7.5.1, 3.5)	59m 59s
check (macos-11.0)	8m 4s
test (5.3.1, 2.3)	4h 32m 18s
check (windows-2019, cmake-build,libz-static,curl-static, cmake-build,libz-static,curl-static)	11m 58s
test (5.0.3, 2)	3h 25m 57s
check (ubuntu-20.04, tracing)	10m 12s
test (4.1.3, 1.1)	3h 8m 29s
check (ubuntu-20.04, cmake-build,ssl-vendored,gssapi-vendored,libz-static,curl-static,zstd, cmake...	9m 2s
	12h 57m 21s

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