Update unit_tests.yml to remove Swift 5.9 job #29

Job	Run time
Cxx interop / Construct Cxx interop matrix	3s
Unit tests / Unit tests (5.10)	32s
Unit tests / Unit tests (6.0)	35s
Unit tests / Unit tests (6.1)	46s
Unit tests / Unit tests (nightly-6.1)	36s
Unit tests / Unit tests (nightly-main)	56s
Soundness / API breakage check	1m 51s
Soundness / Broken symlinks check	3s
Soundness / License headers check	3s
Soundness / Documentation check	1m 27s
Soundness / Unacceptable language check	3s
Soundness / Format check	31s
Soundness / Shell check	32s
Soundness / Python lint check	7s
Soundness / YAML lint check	3s
Cxx interop / Cxx interop / Linux (5.10)	1m 12s
Cxx interop / Cxx interop / Linux (6.0)	1m 19s
Cxx interop / Cxx interop / Linux (6.1)	1m 22s
Cxx interop / Cxx interop / Linux (nightly-next)	1m 51s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 36s
	15m 28s

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