Update release v0.26.3 (#6475)

# Description

patching release version 0.26.3 

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### Summary
:arrow_up::bug::sparkles:

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Update the supported version of the `0.26` channel to `v0.26.3` in
`versions.yaml`. This aligns the channel with the latest release of the
radius project.

> _`0.26` channel_
> _Updated to the latest patch_
> _Autumn bugs are fixed_

### Walkthrough
* Update the supported version of the 0.26 channel to v0.26.3
([link](https://github.com/radius-project/radius/pull/6475/files?diff=unified&w=0#diff-1c4cd801df522f4a92edbfb0fea95364ed074a391ea47c284ddc078f512f7b6aL3-R3))

versions.yaml

@@ -1,6 +1,6 @@
 supported:
   - channel: '0.26'
-    version: 'v0.26.2'
+    version: 'v0.26.3'
 deprecated:
   - channel: '0.25'
     version: 'v0.25.0'