Release 0.26: RC release candidate (#6455)

# Description

Creating RC release for 0.26.0

## Type of change

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Fixes: #issue_number

## Auto-generated summary

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### <samp>🤖 Generated by Copilot at 398339b</samp>

### Summary
:sparkles::wrench::package:

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1. :sparkles: - This emoji represents the addition of a new feature or
enhancement, which in this case is the availability of a new release
candidate for radius users to try out.
2. :wrench: - This emoji represents the maintenance or improvement of
existing functionality, which in this case is the update of the
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Update `versions.yaml` with the latest release candidate for radius
0.26. This allows users to install and test the new features and bug
fixes in the radius CLI tool.

> _`versions.yaml` changed_
> _New release candidate added_
> _CLI tool rejoices_

### Walkthrough
* Add a new entry for radius 0.26 release candidate to `versions.yaml`
([link](https://github.com/radius-project/radius/pull/6455/files?diff=unified&w=0#diff-1c4cd801df522f4a92edbfb0fea95364ed074a391ea47c284ddc078f512f7b6aR2-R3))

versions.yaml

@@ -1,4 +1,6 @@
 supported:
+  - channel: '0.26'
+    version: 'v0.26.0-rc1'
   - channel: '0.25'
     version: 'v0.25.0'
 deprecated: