Use GitHub [!IMPORTANT] alert syntax for deprecation note

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: denelon

doc/specs/#3483 - UserMessages.md

@@ -443,7 +443,8 @@ Messages in locale manifests override those in the defaultLocale manifest on a p
 
 The timeline for each phase is driven by client adoption telemetry, not fixed dates. Phase 4 (removal) is explicitly deferred to a future specification.
 
-> **Important:** There is no intent to make breaking changes in WinGet 1.X. The WinGet CLI will continue to function normally with manifests that use `InstallationNotes`, regardless of deprecation phase. The practical driver for deprecation is the **WinGet Community Repository** (winget-pkgs) schema version policy. The community repository requires new manifest submissions to use schema version **n** or **n-1** (the current or immediately prior version). As the repository moves forward to schema versions where `UserMessages` is the expected field, publishers and manifest authors will be required to adopt the new field and stop using `InstallationNotes` as a condition of submitting to the community repository — not because the client rejects it, but because the repository's validation pipeline enforces the current schema versions.
+> [!IMPORTANT]
+> There is no intent to make breaking changes in WinGet 1.X. The WinGet CLI will continue to function normally with manifests that use `InstallationNotes`, regardless of deprecation phase. The practical driver for deprecation is the **WinGet Community Repository** (winget-pkgs) schema version policy. The community repository requires new manifest submissions to use schema version **n** or **n-1** (the current or immediately prior version). As the repository moves forward to schema versions where `UserMessages` is the expected field, publishers and manifest authors will be required to adopt the new field and stop using `InstallationNotes` as a condition of submitting to the community repository — not because the client rejects it, but because the repository's validation pipeline enforces the current schema versions.
 
 ## Future Considerations