Remove deprecation of withGracefulShutdownHandler if compiler < 6 #20

Job	Run time
Cxx interop / Construct Cxx interop matrix	2s
Unit tests / Construct unit test matrix	2s
Soundness / Unacceptable language check	4s
Soundness / Documentation check	1m 12s
Soundness / API breakage check	1m 1s
Soundness / Broken symlinks check	4s
Soundness / Format check	31s
Soundness / License headers check	5s
Soundness / Shell check	35s
Soundness / YAML lint check	2s
Soundness / Python lint check	9s
Unit tests / Unit tests / Linux (5.9)	49s
Unit tests / Unit tests / Linux (5.10)	47s
Unit tests / Unit tests / Linux (6.0)	1m 1s
Unit tests / Unit tests / Linux (nightly-6.0)	1m 4s
Unit tests / Unit tests / Linux (nightly-main)	1m 2s
Cxx interop / Cxx interop / Linux (5.9)	49s
Cxx interop / Cxx interop / Linux (5.10)	52s
Cxx interop / Cxx interop / Linux (6.0)	1m 0s
Cxx interop / Cxx interop / Linux (nightly-6.0)	1m 0s
Cxx interop / Cxx interop / Linux (nightly-main)	59s
	13m 10s

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