docs: release: Add note on boot serial extension rework #1568

Job	Run time
environment (esp32, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secureb...	2m 54s
environment (esp32, serialrecovery)	2m 2s
environment (esp32s2, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	3m 5s
environment (esp32s2, serialrecovery)	1m 54s
environment (esp32s3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	2m 11s
environment (esp32s3, serialrecovery)	2m 6s
environment (esp32c3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	2m 28s
environment (esp32c3, serialrecovery)	2m 19s
environment (esp32, multi-image,multi-boot, multi)	2m 2s
environment (esp32s3, multi-image,multi-boot, multi)	2m 43s
	23m 44s

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