Update prereqs for playground if needed

[shainaraskas]

Type of issue

Inaccurate

What documentation page is affected

https://www.elastic.co/docs/solutions/search/rag/playground

What happened?

the playground docs still show Gemini 1.5 as the LLMs customers should use when creating a connector

However, according to Google and one of our public KBs, these models have been deprecated and now users should use Gemini 2.5

validate the specific prereqs for playground and validate whether the prereqs should be altered

Additional info

reported by @Z3r0KooI in slack