submodule: bump third_party/openthread/repo from `f75e2bb` to `a0b9ac7` #3744

Job	Run time
Border Router (mDNSResponder)	24m 29s
Border Router (Avahi)	23m 54s
Border Router TREL (mDNSResponder)	23m 31s
Border Router TREL (Avahi)	23m 36s
Border Router MATN (mDNSResponder)	16m 57s
Border Router NAT64 (mDNSResponder)	22m 26s
Backbone Router	15m 50s
Border Router TREL with FEATURE_FLAG (avahi)	23m 48s
	2h 54m 31s

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