Update default OpenAI moderation model from `text-moderation-latest` to `omni-moderation`

[jacquicollier2-eng]

OpenAI are removing the text-moderation-latest moderation model on 27th October 2025.

This is the default moderation model being used by OpenAI in Spring AI.

It needs to be updated to use omni-moderation instead.

Please see https://platform.openai.com/docs/deprecations#2025-04-28-text-moderation

[ilayaperumalg]

@jacquicollier2-eng Thanks for the catch and reporting! Would you be interested in submitting a PR to address this?

[jacquicollier2-eng]

Hi @ilayaperumalg - I've given it a go - its my first commit to any Spring repo, so please let me know if I've missed anything :-)