deps(amazonq): bump mynahui to 4.21.4 #6313

Bump MynahUI to version 4.21.4 from 4.21.2. This includes two bug
fixes and one feature, as described by the changelog commits and by the
[MynahUI release notes](https://github.com/aws/mynah-ui/releases).

Author: Jurredr

package-lock.json

@@ -0,0 +1,4 @@
+{
+	"type": "Bug Fix",
+	"description": "Fix context menu displaying when typing @, even though input is disallowed"
+}

packages/amazonq/.changes/next-release/Bug Fix-8434a098-361d-4e8d-9de4-c616da38ccbf.json

@@ -0,0 +1,4 @@
+{
+	"type": "Bug Fix",
+	"description": "Up/down history navigation only triggering on first/last line of prompt input"
+}

packages/amazonq/.changes/next-release/Bug Fix-acf80397-e84d-430e-b7e2-fec67736338b.json

@@ -0,0 +1,4 @@
+{
+	"type": "Feature",
+	"description": "Amazon Q: new code syntax highlighter for improved accuracy"
+}

packages/amazonq/.changes/next-release/Feature-6e3400cd-def4-4a63-b344-fd7ffa7fa509.json

@@ -508,7 +508,7 @@
         "@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": "^4.21.3",
+        "@aws/mynah-ui": "^4.21.4",
         "@gerhobbelt/gitignore-parser": "^0.2.0-9",
         "@iarna/toml": "^2.2.5",
         "@smithy/middleware-retry": "^2.3.1",