Deprecate ember-fetch #1521

Job	Run time
Does PR add RFCs?	6s
RFC must be in only one stage before merging (labels)	1s
Stage must be 'Exploring' (via label) for new RFC before merging	1s
Frontmatter stage must be 'accepted' before merging	15s
Filename matches RFC number	15s
Verify Accepted PR URL is correct	13s
Only one RFC can be added in a PR	1s
	52s

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