drop support for EOL node < 18 #321

Job	Run time
Tests	4m 36s
Percy Tests	3m 26s
Floating Dependencies	2m 31s
ember-lts-3.16	4m 2s
ember-canary - Allowed to fail	3m 50s
ember-lts-3.20	4m 19s
ember-release-no-deprecations - Allowed to fail	3m 21s
ember-lts-3.24	3m 20s
ember-lts-3.28	3m 8s
ember-release	5m 4s
ember-beta	3m 17s
ember-classic	2m 59s
ember-default-with-jquery	3m 33s
embroider-safe	3m 13s
embroider-optimized	4m 27s
no-deprecations	3m 8s
	58m 14s

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