Advance RFC #0389 `"Dynamic tag names in glimmer templates."` to Stage Ready for Release #1254

Job	Run time
Does PR add RFCs?	9s
Stage must be 'Exploring' (via label) for new RFC before merging	0s
Only one RFC can be added in a PR	0s
RFC must be in only one stage before merging (labels)	0s
Filename matches RFC number	0s
Verify Accepted PR URL is correct	0s
Frontmatter stage must be 'accepted' before merging	0s
	9s

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