maint: Update project status and add status link in README (#379)

## Which problem is this PR solving?
Updates project status and add link to sunset issue to README.

## Short description of the changes
- Update project status in `OSSMETADATA` to maintained
- Add link to sunset issue in README
  - #378

OSSMETADATA

@@ -1 +1 @@
-osslifecycle=active
+osslifecycle=maintained

README.md

@@ -5,6 +5,8 @@
 [![Dependencies](https://img.shields.io/librariesio/release/nuget/Honeycomb.Opentelemetry.svg)](https://github.com/honeycombio/honeycomb-opentelemetry-dotnet/blob/main/src/Honeycomb.OpenTelemetry/Honeycomb.OpenTelemetry.csproj)
 [![Nuget](https://img.shields.io/nuget/v/Honeycomb.OpenTelemetry.svg)](https://www.nuget.org/packages/Honeycomb.OpenTelemetry)
 
+**STATUS**: This project is being Sunset. See [this issue](https://github.com/honeycombio/honeycomb-opentelemetry-dotnet/issues/193) for more details.
+
 This is Honeycomb's distribution of OpenTelemetry for .NET.
 It makes getting started with OpenTelemetry and Honeycomb easier!