Revert "rdkafka-sys: upgrade to librdkafka v2.3.0" #21

Job	Run time
check-minimal-versions	3m 32s
lint	3m 40s
test (5.3.1, 2.3)	4m 50s
test (5.0.3, 2)	21m 55s
check (macos-11.0)	7m 39s
test (4.1.3, 1.1)	4m 40s
check (windows-2019, cmake-build,libz-static,curl-static, cmake-build,libz-static,curl-static)	8m 1s
check (ubuntu-20.04, tracing)	4m 36s
check (ubuntu-20.04, cmake-build,ssl-vendored,gssapi-vendored,libz-static,curl-static,zstd, cmake...	5m 44s
	1h 4m 37s

Provide feedback