Update to `librdkafka` 2.8.0 #55

Job	Run time
Construct Cxx interop matrix	5s
Unit tests / Unit tests (5.9)	4m 10s
Unit tests / Unit tests (5.10)	4m 1s
Unit tests / Unit tests (6.0)	2m 59s
Unit tests / Unit tests (nightly-6.1)	3m 8s
Unit tests / Unit tests (nightly-main)	4m 35s
Soundness / API breakage check	3m 23s
Soundness / Documentation check	1m 45s
Soundness / Unacceptable language check	10s
Soundness / License headers check	5s
Soundness / Broken symlinks check	9s
Soundness / Format check	33s
Soundness / Shell check	33s
Soundness / YAML lint check	5s
Soundness / Python lint check	7s
Cxx interop / Linux (5.9)	1m 21s
Cxx interop / Linux (5.10)	1m 28s
Cxx interop / Linux (6.0)	1m 32s
Cxx interop / Linux (nightly-6.1)	2m 11s
Cxx interop / Linux (nightly-main)	1m 47s
	34m 7s

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