Fix(Core): move command to 'plugins' namespace

[stonebuzz]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes GLPI 11.0.1 issues PHP Warnings regarding fields plugin #1033

Prevent PHP warning

PHP User Warning (512): Plugin command `plugin:fields:check_database` must be moved in the `plugins:fields` namespace. at CommandLoader.php line 273 in ./src/Glpi/Console/CommandLoader.php at line 273
PHP User Warning (512): Plugin command `plugin:fields:regenerate_files` must be moved in the `plugins:fields` namespace. at CommandLoader.php line 273 in ./src/Glpi/Console/CommandLoader.php at line 273

Screenshots (if appropriate):