[child] update MLR masks when removing registered IPv6 addresses

[abtink]

This commit modifies Child::RemoveIp6Address() to update the mMlrToRegisterMask and mMlrToUnregisterMask when a registered child IPv6 address entry is removed from the list. These bit-vector masks track the MLR state associated with the entry using its index in the array. Since Array::Remove() replaces the deleted entry with the last one in the array, the MLR masks are also updated to reflect this change.

---

This PR contains the commit from #10422. Please check and review the last commit. Thanks.

Fixes the issue mentioned here -> #10422 (comment)

[size-report]

Size Report of OpenThread

Merging #10425 into main(8bd427b).

name	branch	text	data	bss	total
ot-cli-ftd	main	466232	856	66348	533436
	#10425	466344	856	66348	533548
	+/-	+112	0	0	+112
ot-ncp-ftd	main	435396	760	61576	497732
	#10425	435508	760	61576	497844
	+/-	+112	0	0	+112
libopenthread-ftd.a	main	235489	95	40286	275870
	#10425	235605	95	40286	275986
	+/-	+116	0	0	+116
libopenthread-cli-ftd.a	main	57574	0	8075	65649
	#10425	57574	0	8075	65649
	+/-	0	0	0	0
libopenthread-ncp-ftd.a	main	32127	0	5924	38051
	#10425	32127	0	5924	38051
	+/-	0	0	0	0
ot-cli-mtd	main	364312	760	51212	416284
	#10425	364312	760	51212	416284
	+/-	0	0	0	0
ot-ncp-mtd	main	347108	760	46456	394324
	#10425	347108	760	46456	394324
	+/-	0	0	0	0
libopenthread-mtd.a	main	157814	0	25166	182980
	#10425	157814	0	25166	182980
	+/-	0	0	0	0
libopenthread-cli-mtd.a	main	39785	0	8059	47844
	#10425	39785	0	8059	47844
	+/-	0	0	0	0
libopenthread-ncp-mtd.a	main	25007	0	5924	30931
	#10425	25007	0	5924	30931
	+/-	0	0	0	0
ot-cli-ftd-br	main	550112	864	131188	682164
	#10425	550240	864	131188	682292
	+/-	+128	0	0	+128
libopenthread-ftd-br.a	main	323836	100	105102	429038
	#10425	323952	100	105102	429154
	+/-	+116	0	0	+116
libopenthread-cli-ftd-br.a	main	71468	0	8099	79567
	#10425	71468	0	8099	79567
	+/-	0	0	0	0
ot-rcp	main	62280	564	20636	83480
	#10425	62280	564	20636	83480
	+/-	0	0	0	0
libopenthread-rcp.a	main	9734	0	5060	14794
	#10425	9734	0	5060	14794
	+/-	0	0	0	0
libopenthread-radio.a	main	18939	0	222	19161
	#10425	18939	0	222	19161
	+/-	0	0	0	0