text: Title: [BUG] Removing available option for custom field (of type select) results in undesired data change for many existing documents.
  Body: ### Description
  
  It looks like the value of custom select field is stored by it's options index and not value. If we remove any option from the middle of the list, values for this custom field are altered for documents, that have current value of this field set to option with higher index that the removed one.
  
  
  ### Steps to reproduce
  
  1. Create custom field of type select.
  2. Add 3 options - "a", "b", and "c".
  3. Add document, assign custom field, pick option "b".
  4. Edit custom field and remove option "a".
  
  Now the document has value "c" instead of "b" for this custom field.
  
  ### Webserver logs
  
  ```bash
  N/A
  ```
  
  
  ### Browser logs
  
  _No response_
  
  ### Paperless-ngx version
  
  2.13.5
  
  ### Host OS
  
  Ubuntu 22.04.5
  
  ### Installation method
  
  Bare metal
  
  ### System status
  
  _No response_
  
  ### Browser
  
  _No response_
  
  ### Configuration changes
  
  _No response_
  
  ### Please confirm the following
  
  - [X] I believe this issue is a bug that affects all users of Paperless-ngx, not something specific to my installation.
  - [X] This issue is not about the OCR or archive creation of a specific file(s). Otherwise, please see above regarding OCR tools.
  - [X] I have already searched for relevant existing issues and discussions before opening this report.
  - [X] I have updated the title field above with a concise description.