submodule: bump third_party/openthread/repo from `bf587dd` to `c73829… #569

Job	Run time
Border Router (mDNSResponder)	24m 30s
Border Router (Avahi)	24m 23s
Border Router TREL (mDNSResponder)	23m 49s
Border Router TREL (Avahi)	24m 0s
Border Router MATN (mDNSResponder)	17m 13s
Border Router NAT64 (mDNSResponder)	22m 5s
Backbone Router	15m 50s
Border Router TREL with FEATURE_FLAG (avahi)	23m 35s
	2h 55m 25s

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