CI/CD

Switch default primary branch to `main`, update docs from Travis to G… #1938

Job	Run time
Lint files	19s
Test compatibility (ember-lts-4.4)	1m 6s
Test addon (locked dependencies) (ember)	48s
Test addon (floating dependencies) (ember)	58s
Test compatibility (ember-lts-4.8)	1m 3s
Test addon (locked dependencies) (node)	25s
Test addon (floating dependencies) (node)	35s
Test compatibility (ember-lts-4.12)	1m 3s
Test addon (locked dependencies) (test-apps)	40s
Test addon (floating dependencies) (test-apps)	52s
Test compatibility (ember-lts-5.4)	1m 8s
Test compatibility (ember-release)	1m 1s
Test compatibility (ember-beta)	1m 3s
Test compatibility (ember-canary)	1m 0s
Test compatibility (embroider-safe)	1m 5s
Deploy app	5s
	13m 11s

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