Update release v0.26.4 (#6499)

# Description

Patching 0.26.4 release

## Type of change

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Fixes: #issue_number

## Auto-generated summary

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### Summary
📝🚀🗑️

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This pull request prepares the documentation for the Radius v0.26.4
patch release. It adds a new release notes section and updates the
`versions.yaml` file.

> _`Radius` has a patch_
> _New section in release notes_
> _Autumn of updates_

### Walkthrough
* Add a new section to the release notes document for Radius v0.26.4
([link](https://github.com/radius-project/radius/pull/6499/files?diff=unified&w=0#diff-e35910d59c0b96639a85b3cad03249f5ea3323d5da338626994b887126dc20d0R1-R6))
* Update the `versions.yaml` file to reflect the latest supported
version of Radius (v0.26.4) and the deprecated version (v0.25)
([link](https://github.com/radius-project/radius/pull/6499/files?diff=unified&w=0#diff-1c4cd801df522f4a92edbfb0fea95364ed074a391ea47c284ddc078f512f7b6aL3-R3))

docs/release-notes/v0.26.4.md

@@ -0,0 +1,23 @@
+## Radius v0.26.4
+
+This release fully switches us to Github assets in preparation for open-sourcing. Check out the [changelog](#changelog) for more details of what was addressed in this patch.
+
+## Changelog
+
+* Release 0.26.0 by @sk593 in https://github.com/radius-project/radius/pull/6458
+* Release 0.26.1 by @sk593 in https://github.com/radius-project/radius/pull/6460
+* Pagination causing Dev Recipes to not be installed and Controller Pod permission fix by @ytimocin in https://github.com/radius-project/radius/pull/6466
+* patch release v0.26.2 by @sk593 in https://github.com/radius-project/radius/pull/6470
+* Handle 404 as success in recipe deletion by @willdavsmith in https://github.com/radius-project/radius/pull/6467
+* Update release v0.26.3 by @sk593 in https://github.com/radius-project/radius/pull/6475
+* Add Deployment reconciler by @rynowak in https://github.com/radius-project/radius/pull/6444
+* Make application and environment configurable by @rynowak in https://github.com/radius-project/radius/pull/6480
+* Add functional test for Kubernetes interop by @rynowak in https://github.com/radius-project/radius/pull/6488
+* Add `graph` alias for `rad app connections` by @AaronCrawfis in https://github.com/radius-project/radius/pull/6489
+* Update README.md by @jonvsm in https://github.com/radius-project/radius/pull/6482
+* Fix errata in Kubernetes controllers by @rynowak in https://github.com/radius-project/radius/pull/6494
+* Migrate to GHCR by @willdavsmith in https://github.com/radius-project/radius/pull/6478
+* Update installation scripts by @sk593 in https://github.com/radius-project/radius/pull/6495
+* Add `ok-to-test` trigger for functional tests by @willdavsmith in https://github.com/radius-project/radius/pull/6481
+
+**Full Changelog**: https://github.com/radius-project/radius/compare/v0.26.0...v0.26.4

versions.yaml

@@ -1,6 +1,6 @@
 supported:
   - channel: '0.26'
-    version: 'v0.26.3'
+    version: 'v0.26.4'
 deprecated:
   - channel: '0.25'
     version: 'v0.25.0'