Update Ktor version to 3.1.1 to fix dependency issue

Updated the Ktor version in `gradle/libs.versions.toml` to 3.1.1. This addresses the reported issue where the Vertex AI SDK was depending on an older Ktor version, causing conflicts for consumers using newer Ktor versions. Verified that the project builds and tests pass with Ktor 3.1.1.

Author: google-labs-jules[bot]

gradle/libs.versions.toml

@@ -45,7 +45,7 @@ jsonassert = "1.5.0"
 kotest = "5.9.0" # Do not use 5.9.1 because it reverts the fix for https://github.com/kotest/kotest/issues/3981
 kotestAssertionsCore = "5.8.1"
 kotlin = "2.0.21"
-ktorVersion = "3.0.3"
+ktorVersion = "3.1.1"
 legacySupportV4 = "1.0.0"
 lifecycleProcess = "2.3.1"
 material = "1.12.0"