Use only `FoundationEssentials` when possible #48

Job	Run time
Semantic Version label check	5s
Unit tests / Construct unit test matrix	4s
Soundness / Broken symlinks check	3s
Soundness / API breakage check	1m 12s
Soundness / Documentation check	34s
Soundness / Unacceptable language check	3s
Soundness / License headers check	3s
Soundness / Format check	32s
Soundness / Python lint check	6s
Soundness / Shell check	30s
Soundness / YAML lint check	2s
Swift 6 Language Mode / Swift 6 language mode	56s
Unit tests / Unit tests / Linux (6.0)	57s
Unit tests / Unit tests / Linux (nightly-6.0)	1m 3s
Unit tests / Unit tests / Linux (nightly-main)	1m 6s
	7m 16s

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