modify oracle lexer name Fixes #15106

[engchina]

Summary

Change default oracle lexer from "CHINESE_VGRAM_LEXER" to "WORLD_LEXER".
Change default name form "multilingual_lexer" to "world_lexer" to avoid confusing.
Cause there are WORLD_LEXER and MULTI_LEXER, compared them, I think WORLD_LEXER is better than MULTI_LEXER.

Oralce docs about WORLD_LEXER and MULTI_LEXER for reference.

WORLD_LEXER:
https://docs.oracle.com/en/database/oracle/oracle-database/21/ccref/oracle-text-indexing-elements.html#GUID-FA7DB079-3247-49DB-A9F9-067F6281F879

MULTI_LEXER:
https://docs.oracle.com/en/database/oracle/oracle-database/21/ccref/oracle-text-indexing-elements.html#GUID-93B17833-A046-468C-BE5F-2BD09A61935C

Fixes #15106

Checklist

Important

Please review the checklist below before submitting your pull request.

• [] This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods