Problem: There are still outdated mentions of Flash on some doc pages

## Affected versions

2.7, 2.8

## Affected pages

* First paragraph of this section: https://www.accesstomemory.org/docs/2.8/admin-manual/maintenance/troubleshooting/#why-can-t-i-upload-large-digital-objects
* In the description of what FFMPEG is used for: https://www.accesstomemory.org/docs/2.8/admin-manual/installation/requirements/#other-dependencies

## Error description

AtoM's Flash-based multi-uploader was removed in [release 2.6.3](https://wiki.accesstomemory.org/Releases/Release_announcements/Release_2.6.3) after support for Flash in web browsers was fully discontinued. Unfortunately, there are still a couple places where we mention the use of Flash, or the creation of Flash-based derivatives, that are no longer accurate.

## Suggested fix

Update these sections so that they no longer mention Flash

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## Notes

Doing a docs search does reveal the mention of Flash in two other places, but I believe these are fine for now.

The first is a warning for upgrading users, specifically about the removal of Flash support and the need to regenerate video derivatives if upgrading from release 2.6.2 or earlier:

* https://www.accesstomemory.org/docs/2.8/admin-manual/installation/upgrading/#regenerate-the-digital-object-reference-and-thumbnail-images-optional

This is fine for now, but perhaps can be removed after the 2.9 release or something? No rush anyway - after all, there are still ICA-AtoM 1.x users in the wild, so 2.6 may be in use by some for a while to come.

The second is a listing of supported file formats when processing digital objects. This table remains accurate, and though instances of Flash based digital objects will continue to decline, it does no harm to mention them here for now. See:

* https://www.accesstomemory.org/docs/2.8/user-manual/import-export/upload-digital-object/#files-formats

Open the two affected sections in the linked AtoM 2.8 troubleshooting and installation requirements documentation pages and review their current wording. Remove the outdated Flash references while leaving the separately noted upgrade warning and supported file-format entry unchanged; the documentation is done when those two sections no longer describe Flash use or Flash-based derivatives.

Written by the indexing model from the issue text.