Deprecate `external` field in Releases and Release Links APIs

Overview

As a result of #333967 (closed), all release links will be treated as external moving on.

Therefore, we want to deprecate the external field in Releases and Release Links APIs.

The field will be removed in %16.0.

Proposal

• Announce the deprecation in the deprecated feature removal schedule.
• Deprecate the field in Releases API.
• Deprecate the field in Release Links API.