Update unit_tests.yml to remove Swift 5.9 job #30

Job	Run time
Cxx interop / Construct Cxx interop matrix	5s
Unit tests / Unit tests (5.10)	1m 29s
Unit tests / Unit tests (6.0)	1m 34s
Unit tests / Unit tests (6.1)	1m 27s
Unit tests / Unit tests (nightly-6.1)	1m 58s
Unit tests / Unit tests (nightly-main)	1m 54s
Soundness / API breakage check	1m 45s
Soundness / Documentation check	1m 31s
Soundness / Unacceptable language check	3s
Soundness / Broken symlinks check	3s
Soundness / License headers check	3s
Soundness / Python lint check	7s
Soundness / Format check	40s
Soundness / YAML lint check	4s
Soundness / Shell check	31s
Cxx interop / Cxx interop / Linux (5.10)	1m 7s
Cxx interop / Cxx interop / Linux (6.0)	1m 24s
Cxx interop / Cxx interop / Linux (6.1)	1m 22s
Cxx interop / Cxx interop / Linux (nightly-next)	1m 35s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 51s
	20m 33s

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