German language pack not available

# Bug Report

## Prerequisites

* [x] Can you reproduce the problem on TYPO3 v11.5 LTS
* [x] Can you reproduce the problem on TYPO3 v12.4 LTS
* [x] Did you [perform a cursory search](https://github.com/issues?q=is%3Aissue+user%3Abenjaminkott+repo%3Abenjaminkott%2Fbootstrap_package) to see if your bug or enhancement is already reported?

## Description

When I try to install the German language pack for bootstrap_package with "Manage Language Packs" I get the following message:

```
Language packs updated
0 new language packs downloaded, 0 language packs updated, 0 language packs skipped, 1 language pack not available
```

The download of language packs for system extensions and other extensions are working.

File on translation server seems to exist, but is outdated (last changes happened 2020-01-10): https://typo3.org/fileadmin/ter/b/o/bootstrap_package-l10n/bootstrap_package-l10n-de.zip

See also #664

Start by reviewing the translation-server archive at the linked bootstrap_package-l10n-de.zip URL and the related issue #664, then trace how TYPO3's Manage Language Packs discovers this package. Confirm the expected package metadata and availability path before checking that the German pack can be downloaded and updated successfully.

Written by the indexing model from the issue text.