Drop Linux 5.8 from the release workflow #129

Job	Run time
Run tests using swift-syntax-dev-utils (Linux)	24m 27s
Validate generated code	2m 57s
Run tests using swift-syntax-dev-utils (Windows)	52m 5s
Test / Linux (5.9 - jammy)	3m 59s
Test / Linux (5.10 - jammy)	3m 53s
Test / Linux (6.0 - jammy)	4m 13s
Test / Windows (5.9 - windows-2022)	18m 57s
Test / Windows (6.0 - windows-2022)	17m 40s
Test / Linux (nightly-main - jammy)	4m 53s
Test / Windows (nightly - windows-2019)	16m 23s
Test / Linux (nightly-6.1 - jammy)	5m 2s
Test / Windows (nightly-6.1 - windows-2019)	16m 15s
Soundness / API breakage check	0s
Soundness / Documentation check	5m 8s
Soundness / Format check	39s
Soundness / License headers check	12s
Soundness / Broken symlinks check	4s
Soundness / Shell check	31s
Soundness / YAML lint check	3s
Soundness / Unacceptable language check	3s
Soundness / Python lint check	5s
	2h 57m 29s

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