Communicate the Build Insights switchover and Build Analysis retirement timeline

## Business value

Repository owners need clear timing and actions so the move to Build Insights is coordinated without disrupting .NET delivery.

## Expected outcome

Once Build Insights meets the primary-tool readiness gate, stakeholders understand when and how to switch their required check, how long Build Analysis will remain available, and when Build Insights becomes the only supported experience.

## Acceptance criteria

- Communication is sent after the readiness decision in [#6650](https://github.com/dotnet/arcade-services/issues/6650).
- Repository owners are asked to replace the Build Analysis check with the Build Insights check.
- The expected switchover timing is communicated as approximately the end of September 2026.
- Build Analysis continues running for at least one month after the switchover.
- Build Insights becomes the only supported tool no earlier than after the .NET 11 release.
- [Build Analysis](https://helix.dot.net/BuildAnalysis) displays a notice directing users to Build Insights and communicating the planned retirement of the Helix experience.
- The communication aligns with the broader adoption guidance in [#6648](https://github.com/dotnet/arcade-services/issues/6648).

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🤖 _Drafted by an AI agent on behalf of @michalpavelka._

Start by reviewing the readiness decision in #6650 and the broader adoption guidance in #6648, then inspect the Build Analysis experience linked in the issue. Done means the switchover communication is sent with the stated timing and support window, repository owners are directed to Build Insights, and Build Analysis displays the retirement notice.

Written by the indexing model from the issue text.