deps(amazonq): update mynah ui to 4.21.0 (#6130)

## Problem
mynah ui has several improvements since we've last updated:
- Fix chat syntax highlighting when using several different themes
- UI improvements to Amazon Q Chat: New splash loader animation, initial
streaming card animation, improved button colours
- Navigate through prompt history by using the up/down arrows

## Solution
Update mynah ui

---

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CONTRIBUTING.md -->

License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: jpinkney-aws

package-lock.json

@@ -0,0 +1,4 @@
+{
+	"type": "Bug Fix",
+	"description": "Fix chat syntax highlighting when using several different themes"
+}

packages/amazonq/.changes/next-release/Bug Fix-17772b9d-7f10-4e16-8b53-0561d6b08f4a.json

@@ -0,0 +1,4 @@
+{
+	"type": "Feature",
+	"description": "UI improvements to Amazon Q Chat: New splash loader animation, initial streaming card animation, improved button colours"
+}

packages/amazonq/.changes/next-release/Feature-3e91ab38-b708-4fcc-888e-7a532c210a69.json

@@ -0,0 +1,4 @@
+{
+	"type": "Feature",
+	"description": "Navigate through prompt history by using the up/down arrows"
+}

packages/amazonq/.changes/next-release/Feature-c0fdc8fe-e0e5-422c-a76a-1d7461460e81.json

@@ -506,7 +506,7 @@
         "@aws-sdk/property-provider": "3.46.0",
         "@aws-sdk/smithy-client": "^3.46.0",
         "@aws-sdk/util-arn-parser": "^3.46.0",
-        "@aws/mynah-ui": "^4.18.1",
+        "@aws/mynah-ui": "^4.21.0",
         "@gerhobbelt/gitignore-parser": "^0.2.0-9",
         "@iarna/toml": "^2.2.5",
         "@smithy/middleware-retry": "^2.3.1",