submodule: bump third_party/openthread/repo from `a0f861d` to `7b72e97` #5370

Job	Run time
Border Router (mDNSResponder)	29m 12s
Border Router (Avahi)	29m 59s
Border Router TREL (mDNSResponder)	28m 59s
Border Router TREL (Avahi)	28m 51s
Border Router MATN (mDNSResponder)	18m 21s
Border Router Internet Access Features (mDNSResponder)	25m 45s
Backbone Router	17m 11s
Border Router TREL with FEATURE_FLAG (avahi)	28m 40s
	3h 26m 58s

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