Project migration does not map the Drupal 7 gettext connector id

Problem

The project migration maps the Drupal 7 connector ids to the 3.0.x plugin ids with a static map that lets unknown values through. It knows the drupal.org connector (both its old and new id) and the two sources of the local packages connector, but not l10n_gettext_uploads, the connector of projects fed by uploaded .po files. Such a project keeps the Drupal 7 id, and loading its connector throws a plugin not found exception wherever the project is used.

Proposed resolution

Map l10n_gettext_uploads to gettext:upload, the connector ported in #3292190, in the module migration and in the site configuration.

Tests

The project migration kernel test migrates a gettext upload project and a local packages project next to the drupal.org one and checks the mapped connector of each. There is nothing to test on Drupal 7 for this, the Drupal 7 connector id is the specification.

LLM disclosure

LLM was used to find, diagnose explain and fix this issue. With human review.