Drop support for Python 3.8. #3585

Job	Run time
smoke (amaranth-lang/amaranth-boards)	25s
test (3.9)	2m 47s
smoke (amaranth-lang/amaranth-stdio)	34s
check-links	49s
test (3.10)	2m 48s
document	51s
smoke (amaranth-lang/amaranth-soc)	34s
test (3.11)	2m 52s
test (3.12)	3m 38s
test (3.13)	3m 13s
test (pypy-3.9)	6m 26s
test (pypy-3.10)	5m 24s
test (3.14-dev)	2m 1s
publish-docs	13s
publish-docs-dev	0s
required	1s
publish-package	44s
publish-schemas	13s
publish-release	0s
	33m 33s

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