#18958: fix: replace deprecated eleven_monolingual_v1 with eleven_flash_v2

Fixes #18895 The `eleven_monolingual_v1` model has been deprecated by ElevenLabs and is no longer available on the free tier. This replaces it with `eleven_flash_v2` in the TTS provider model list. ### Changes - `src/gateway/server-methods/tts.ts`: Replace `eleven_monolingual_v1` with `eleven_flash_v2` in the ElevenLabs model list <!-- greptile_comment --> <h3>Greptile Summary</h3> Replaced the deprecated ElevenLabs TTS model `eleven_monolingual_v1` with `eleven_flash_v2` in the provider model list. - Updated `src/gateway/server-methods/tts.ts:142` to replace `eleven_monolingual_v1` with `eleven_flash_v2` in the ElevenLabs models array - The deprecated model is no longer available on the ElevenLabs free tier - No other references to the deprecated model found in the codebase <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The change is a straightforward model name replacement that addresses a deprecation by the upstream provider. The modification is localized to a single line in a model list array, with no logic changes or side effects. The deprecated model has been completely removed from the codebase with no remaining references. - No files require special attention _{Last reviewed commit: 021ade5} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->