Revert: debian:testing-20240926-slim [ci skip]

This reverts commit 00bbc3f.

Something borken with the image signature:
https://github.com/curl/curl/actions/runs/11067644862/job/30752034560?pr=15065
```
No signatures for debian:testing-20240926-slim

Administrative keys for debian:testing-20240926-slim

  Repository Key:	5717dcd81d9fb5b73aa15f2d887a6a0de543829ab9b2d411acce9219c2f8ba3a
  Root Key:	575d013f89e3cbbb19e0fb06aa33566c22718318e0c9ffb1ab5cc4291e07bf84
No valid trust data for testing-20240926-slim
```

_versions.sh

@@ -3,7 +3,7 @@
 # Copyright (C) Viktor Szakats. See LICENSE.md
 # SPDX-License-Identifier: MIT
 
-export DOCKER_IMAGE='debian:testing-20240926-slim'
+export DOCKER_IMAGE='debian:testing-20240904-slim'
 
 export CURL_VER_='8.10.1'
 export CURL_HASH=73a4b0e99596a09fa5924a4fb7e4b995a85fda0d18a2c02ab9cf134bebce04ee