submodule: bump third_party/openthread/repo from `499f668` to `70adcab` #3308

Job	Run time
Border Router (mDNSResponder)	28m 26s
Border Router (mDNSResponder)	28m 26s
Border Router (Avahi)	29m 34s
Border Router (Avahi)	27m 6s
Border Router TREL (mDNSResponder)	28m 17s
Border Router TREL (mDNSResponder)	26m 55s
Border Router TREL (Avahi)	26m 57s
Border Router TREL (Avahi)	26m 57s
Border Router MATN (mDNSResponder)	21m 5s
Border Router MATN (mDNSResponder)	21m 5s
Border Router NAT64 (mDNSResponder)	26m 17s
Border Router NAT64 (mDNSResponder)	26m 17s
Backbone Router	20m 53s
Backbone Router	20m 53s
Border Router TREL with FEATURE_FLAG (avahi)	29m 6s
Border Router TREL with FEATURE_FLAG (avahi)	29m 6s
	6h 57m 20s

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