submodule: bump third_party/openthread/repo from `84e80c6` to `f6cee7… #5451

Job	Run time
Border Router (mDNSResponder)	30m 33s
Border Router (Avahi)	29m 16s
Border Router TREL (mDNSResponder)	28m 32s
Border Router TREL (Avahi)	29m 11s
Border Router MATN (mDNSResponder)	17m 53s
Border Router Internet Access Features (mDNSResponder)	22m 14s
Backbone Router	19m 28s
Border Router TREL with FEATURE_FLAG (avahi)	31m 54s
	3h 29m 1s

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