Update changelog for build warning cleanup and UnpackStreamAsync API

[KevinJump]

What

Adds two entries to CHANGELOG.md under [Unreleased] → Changed, covering the four PRs merged in this session's build-warning cleanup pass.

Why

The changelog wasn't updated as part of #1002-#1005, and per the repo convention (see the existing ExportContainer entry) extender-facing API changes should be recorded there.

What's recorded

• Extender API: ISyncManagementService.UnpackStreamAsync(Stream) added; UnpackStream(Stream) obsoleted (removed in v19). (Add async UnpackStreamAsync to ISyncManagementService, obsolete sync UnpackStream #1005)
• A combined note on the MasterTemplateAlias → LayoutTemplateAlias swap and inlining the legacy {localLink:x} parsing that Umbraco is removing in v18, explicitly flagged as no behavioural change. (Add missing XML comments to clear CS1591 build warnings #1002, Replace obsolete ITemplate.MasterTemplateAlias with LayoutTemplateAlias #1003, Inline legacy local link parsing to drop obsolete HtmlLocalLinkParser API #1004)

Notes for reviewers

• Docs-only change, no code touched.
• Deliberately didn't give the XML-doc-comment addition (also part of Add missing XML comments to clear CS1591 build warnings #1002) its own line — it's purely internal and not user-facing behaviour, so it's folded into the same bullet as the warning fix it shipped alongside.

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