feat: improve query performance for `find*()`

[datamweb]

Description This Pull Request introduces a database migration that adds an index to the deleted_at column in the users table. This change is intended to enhance the performance of queries that filter based on the deleted_at field, particularly for cases involving soft deletes.

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [x] User guide updated
• [ ] Conforms to style guide

[kenjis]

This needs to be carefully considered. See https://tech-torana-co-jp.translate.goog/entry/2023/11/27/123000?_x_tr_sl=ja&_x_tr_tl=en&_x_tr_hl=ja&_x_tr_pto=wapp

[datamweb]

Hi Kenjis, I did some research on this and it seems like it won’t have a positive impact. So, I’m going to close it. Thanks for the link you shared!"