Spring-ai Adjust the VertexAiGeminiChatModel to make use of the new system-instructions support.

Google Vertex AI Gemini API 26.41 BOM release had introduced system-instructions support: - https://cloud.google.com/vertex-ai/generative-ai/docs/learn/prompts/system-instructions - https://cloud.google.com/vertex-ai/generative-ai/docs/learn/prompts/system-instructions#gemini-system-instructions-code-samples-java

As until now Gemini lacked system messages support, Spring AI use to convert those messages into to-level User messages. Now we can retire this code if favour of using the new GenerativeModel#systemInstructions API