docs: update to document v2.x limited support (#1729)

## Which problem is this PR solving?

- Communicating support level and lifetime for Refinery v2.x.

## Short description of the changes

- Update README and RELEASE_NOTES with support level and end-of-support
date.

Author: robbkidd

README.md

@@ -10,6 +10,9 @@
 For a detailed list of linked pull requests merged in each release, see [CHANGELOG.md](./CHANGELOG.md).
 For more readable information about recent changes, please see [RELEASE_NOTES.md](./RELEASE_NOTES.md).
 
+📝 With the release of v3.0.0, v2.x enters [maintenance](https://docs.honeycomb.io/troubleshoot/product-lifecycle/release-stages/#maintenance) for support until April 1, 2026.
+We recommend [migrating to Refinery 3](https://docs.honeycomb.io/troubleshoot/product-lifecycle/recommended-migrations/upgrade-refinery/).
+
 ## Purpose
 
 Refinery is a tail-based sampling proxy and operates at the level of an entire [trace](https://docs.honeycomb.io/concepts/tracing/). Refinery examines whole traces and intelligently applies sampling decisions to each trace. These decisions determine whether to keep or drop the trace data in the sampled data forwarded to Honeycomb.

RELEASE_NOTES.md

@@ -7,6 +7,9 @@ While [CHANGELOG.md](./CHANGELOG.md) contains detailed documentation and links t
 This is a performance improvement release, focused on memory and CPU efficiency.
 The improvements introduce some backwards breaking changes which are covered in detail below.
 
+📝 With the release of v3.0.0, v2.x enters [maintenance](https://docs.honeycomb.io/troubleshoot/product-lifecycle/release-stages/#maintenance) for support until April 1, 2026.
+We recommend [migrating to Refinery 3](https://docs.honeycomb.io/troubleshoot/product-lifecycle/recommended-migrations/upgrade-refinery/).
+
 ### Performance Improvements
 
 The improvements are the result of reducing the number of data transformations performed during the receipt and retransmission of traces.