Backport of docs: Add legacy api gateway deprecation to Consul release notes...

Backport of docs: Add legacy api gateway deprecation to Consul release notes into release/1.17.x (#20575)

* backport of commit 610892e6



* backport of commit a14018b6a8202d2b91fd275fe9e63f17193f189a

* backport of commit aebdd6b473d9d43aff2b003b76b80b0fccde617f

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Co-authored-by: David Yu <dyu@hashicorp.com>