Update to `librdkafka` 2.8.0 #54

Job	Run time
Construct Cxx interop matrix	3s
Unit tests / Unit tests (5.9)	3m 59s
Unit tests / Unit tests (5.10)	2m 28s
Unit tests / Unit tests (6.0)	4m 24s
Unit tests / Unit tests (nightly-6.1)	4m 28s
Unit tests / Unit tests (nightly-main)	4m 30s
Soundness / API breakage check	3m 14s
Soundness / Documentation check	1m 52s
Soundness / License headers check	4s
Soundness / Unacceptable language check	3s
Soundness / Broken symlinks check	5s
Soundness / Format check	30s
Soundness / Python lint check	5s
Soundness / Shell check	33s
Soundness / YAML lint check	4s
Cxx interop / Linux (5.9)	1m 30s
Cxx interop / Linux (5.10)	1m 21s
Cxx interop / Linux (6.0)	1m 32s
Cxx interop / Linux (nightly-6.1)	1m 57s
Cxx interop / Linux (nightly-main)	2m 26s
	35m 8s

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