Update release version 0.26.6 (#6509)

# Description

Patching release version 0.26.6

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Fixes: #issue_number

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### Summary
🆙🐛✨

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Update the supported version of the `0.26` channel to `v0.26.6` in
`versions.yaml`. This aligns the channel with the latest release of the
radius project.

> _`0.26` channel_
> _Updated to fix and add things_
> _Radius in autumn_

### Walkthrough
* Update the supported version of the 0.26 channel to v0.26.6
([link](https://github.com/radius-project/radius/pull/6509/files?diff=unified&w=0#diff-1c4cd801df522f4a92edbfb0fea95364ed074a391ea47c284ddc078f512f7b6aL3-R3))

versions.yaml

@@ -1,6 +1,6 @@
 supported:
   - channel: '0.26'
-    version: 'v0.26.5'
+    version: 'v0.26.6'
 deprecated:
   - channel: '0.25'
     version: 'v0.25.0'