mu 1.12: helper functions are deprecated

### I confirm that...

- [X] I have searched the [issue tracker](https://github.com/doomemacs/doomemacs/issues), [documentation](https://docs.doomemacs.org), [FAQ](https://docs.doomemacs.org/-/faq), [Discourse](https://discourse.doomemacs.org), and [Google](https://google.com), in case this issue has already been reported/resolved.
- [X] I have read "[How to Debug Issues](https://doomemacs.org/d/how2debug)", and will use it to provide as much information about this issue as possible.
- [X] The issue can be reproduced on the **latest** available commit of Doom.
- [X] The issue can be reproduced on a stable release of Emacs, such as 27, 28, or 29. *(Unstable versions end in .50, .60, or .9x)*

### Expected behavior
[mu4e](https://github.com/djcb/mu/commit/993f16522abe1ebf3e4dc34aebbce32b5e06fa18) deprecated helper function `mu4e--view-gather-mime-parts`. As a consequence, the commands defined in doom's module `+mu4e-view-select-mime-part-action` and `+mu4e-view-open-attachment` no longer work.

### Current behavior

When one of the two commands above is invoked in mu4e-view mode, message `’mu4e-view--mime-parts’ elisp declaration not found.` is triggered.
I suspect `+mu4e-view-select-mime-part-action` could be seamlessly replaced with `mu4e-view-mime-part-action`. I couldn't find an adequate alternative for `+mu4e-view-open-attachment`, and while `mu4e-view-mime-parts` seems to be the designated replacement function, it's not quite a drop-in replacement

### Steps to reproduce

1. Open mu4e
2. select any email with attachments
3. press A or o

### System Information

N/A, Emacs 29.3 on latest commit