Release Notes aren't always in an SDK-Only Markdown File

[ellahathaway]

The latest 9.0 source-build release announcement attempted to use the following URL for the release notes: https://github.com/dotnet/core/blob/main/release-notes/9.0/9.0.1/9.0.102.md.

However, since 9.0.102 was the only SDK version being released, the actual release notes were at https://github.com/dotnet/core/blob/main/release-notes/9.0/9.0.1/9.0.1.md.

The runtime release notes always include the highest SDK being released. If the 9.0.1 runtime had been released alongside a 9.0.201 SDK and the 9.0.102 SDK, the 9.0.1 runtime markdown would've included the 9.0.201 SDK, and the https://github.com/dotnet/core/blob/main/release-notes/9.0/9.0.1/9.0.102.md file would've existed.

We need to either adjust the release pipeline to support this scenario, or we need to rely on manually updating the release announcement in the instances where the 9.0.1 SDK is the only SDK version being released.

[ellahathaway]

This is being completed as part of my work for dotnet/dotnet#1481

[ellahathaway]

PR: https://dev.azure.com/dnceng/internal/_git/dotnet-release/pullrequest/52065