Make `x25519` the default key-share (previously X25519MLKEM768) when using BC #387

Job	Run time
Build & core Tests (Linux, self-hosted)	4m 3s
Build & core Tests (Linux, self-hosted)	4m 3s
Build & core Tests (Linux, self-hosted)	2m 58s
Build & core Tests (Linux, self-hosted)	3m 55s
Build & core Tests (Linux, self-hosted)	3m 58s
Build & core Tests (Windows, self-hosted)	4m 22s
Build & core Tests (Windows, self-hosted)	2m 45s
Build & core Tests (Windows, self-hosted)	2m 45s
Build & core Tests (Windows, self-hosted)	2m 45s
Build & core Tests (Windows, self-hosted)	2m 45s
Build & core Tests (macOS, self-hosted)	6m 40s
Build & core Tests (macOS, self-hosted)	6m 40s
Build & core Tests (macOS, self-hosted)	6m 40s
Build & core Tests (macOS, self-hosted)	6m 40s
Build & core Tests (macOS, self-hosted)	6m 40s
	1h 7m 39s

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