chore: upgrade functional test suite to Lean version 4.15.0 #170

Job	Run time
lake-build-args	16s
auto-config-false	36s
lake-init-failure	9s
detect-mathlib	1m 24s
lake-init-success (standalone)	9s
auto-config-true	11s
lake-init-success (mathdep math)	1m 15s
lake-check-test-failure	9s
lake-init-success (tomltest .toml)	14s
lake-lint-failure	10s
lake-lint-success	14s
lake-test-failure	11s
lake-test-success	11s
macos-runner	16s
subdirectory-lake-package	10s
windows-runner	35s
	6m 10s

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