Add GitHub action that automatically creates a PR to merge main into a release branch #163

Job	Run time
Run tests using swift-syntax-dev-utils (Linux)	32s
Run tests using swift-syntax-dev-utils (Windows)	34s
Validate generated code	35s
Test / Linux (5.9 - jammy)	35s
Test / Linux (5.10 - jammy)	34s
Test / Linux (6.0 - jammy)	34s
Test / Linux (6.1 - jammy)	35s
Test / Linux (nightly-main - jammy)	33s
Test / Linux (nightly-6.1 - jammy)	35s
Test / Windows (5.9 - windows-2022)	31s
Test / Windows (6.0 - windows-2022)	30s
Test / Windows (6.1 - windows-2022)	32s
Test / Windows (nightly - windows-2019)	34s
Test / Windows (nightly-6.1 - windows-2019)	30s
Soundness / API breakage check	0s
Soundness / Documentation check	35s
Soundness / License headers check	10s
Soundness / Unacceptable language check	4s
Soundness / Broken symlinks check	3s
Soundness / Format check	34s
Soundness / Shell check	31s
Soundness / YAML lint check	2s
Soundness / Python lint check	5s
	9m 48s

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