[repo] misc: rename .cursorignore to .cursorindexingignore (#76494)

### Why?

As of Cursor
[v0.46](https://www.cursor.com/changelog/agent-is-ready-and-ui-refresh),
the `.cursorignore` file also skips AI generation in addition to
disabling indexing. To disable indexing only, should use
`.cursorindexingignore`.

I don't think we want to use `.cursorignore` for disabling AI
generation, so I just renamed it to `.cursorindexingignore`.

Author: devjiwonchoi