Drop support for Python 3.8. #3584

Job	Run time
test (3.9)	2m 26s
test (3.10)	2m 22s
document	48s
check-links	52s
test (3.11)	3m 0s
test (3.12)	4m 8s
smoke (amaranth-lang/amaranth-boards)	26s
test (3.13)	3m 21s
test (pypy-3.9)	7m 1s
test (pypy-3.10)	4m 32s
test (3.14-dev)	1m 44s
smoke (amaranth-lang/amaranth-stdio)	33s
smoke (amaranth-lang/amaranth-soc)	30s
publish-docs	4s
publish-docs-dev	0s
required	1s
publish-package	43s
publish-schemas	5s
publish-release	0s
	32m 36s

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