Bump to Node.js 20 by default #603

Job	Run time
base_setup_pre	38s
base_setup (ubuntu-latest, 3.8)	30s
binder_link	24s
base_setup (ubuntu-latest, 3.11)	26s
base_setup_minimum	40s
base_setup (windows-latest, 3.8)	1m 8s
check_links	3m 14s
base_setup (windows-latest, 3.11)	1m 19s
downstream_defaults	57s
downstream_overrides	2m 6s
base_setup (macos-latest, 3.8)	57s
base_setup (macos-latest, 3.11)	40s
install_minimums (3.8)	37s
base_setup (ubuntu-latest, 3.12)	28s
pr_script	24s
Test Lint	1m 1s
update_snapshots	38s
update_snapshots-manual-server	1m 2s
Test coverage (ubuntu-latest)	34s
Make SDist	1m 19s
Test coverage (windows-latest)	1m 27s
Install from SDist and Test	33s
Combine & check coverage	11s
tests_check	1s
	21m 14s

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