Copy release notes from release-18.0 into main

Signed-off-by: Shlomi Noach <[email protected]>

changelog/18.0/18.0.4/changelog.md

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+# Changelog of Vitess v18.0.4
+
+### Bug fixes 
+#### VReplication
+ * [release-18.0] VReplication: Fix workflow update changed handling (#15621) [#15628](https://github.com/vitessio/vitess/pull/15628)
+### CI/Build 
+#### Build/CI
+ * [release-18.0] Update to latest CodeQL (#15530) [#15533](https://github.com/vitessio/vitess/pull/15533)
+ * [release-18.0] Update go.mod go version to 1.21.9 [#15646](https://github.com/vitessio/vitess/pull/15646)
+ * [release-18.0] Updated `golang.org/x/net` [#15650](https://github.com/vitessio/vitess/pull/15650) 
+#### General
+ * [release-18.0] Upgrade to go1.21.9 [#15639](https://github.com/vitessio/vitess/pull/15639)
+### Regression 
+#### Query Serving
+ * [release-18.0] fix: remove keyspace from column during query builder (#15514) [#15516](https://github.com/vitessio/vitess/pull/15516)
+### Release 
+#### General
+ * [release-18.0] Bump to `v18.0.4-SNAPSHOT` after the `v18.0.3` release [#15489](https://github.com/vitessio/vitess/pull/15489)
+### Testing 
+#### VReplication
+ * [release-18.0] VReplication: Fix vtctldclient SwitchReads related bugs and move the TestBasicV2Workflows e2e test to vtctldclient (#15579) [#15583](https://github.com/vitessio/vitess/pull/15583)
+

changelog/18.0/18.0.4/release_notes.md

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+# Release of Vitess v18.0.4
+The entire changelog for this release can be found [here](https://github.com/vitessio/vitess/blob/main/changelog/18.0/18.0.4/changelog.md).
+
+The release includes 8 merged Pull Requests.
+
+Thanks to all our contributors: @app/vitess-bot, @frouioui, @shlomi-noach, @systay
+

changelog/18.0/README.md

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 ## v18.0
+* **[18.0.4](18.0.4)**
+	* [Changelog](18.0.4/changelog.md)
+	* [Release Notes](18.0.4/release_notes.md)
+
 * **[18.0.3](18.0.3)**
 	* [Changelog](18.0.3/changelog.md)
 	* [Release Notes](18.0.3/release_notes.md)