fix: deprecated function utf8_encode

[Rom1-B]

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.
• This change requires a documentation update.

Description

glpiphplog.NOTICE:   *** PHP Deprecated function (8192): Function utf8_encode() is deprecated in /home/ubuntu/Dev/GLPI/10.0-bugfixes/plugins/datainjection/inc/backend.class.php at line 108
  Backtrace :
  plugins/datainjection/inc/backendcsv.class.php:118 PluginDatainjectionBackend::toUTF8()
  plugins/datainjection/inc/backendcsv.class.php:247 PluginDatainjectionBackendcsv::parseLine()
  ...datainjection/inc/clientinjection.class.php:299 PluginDatainjectionBackendcsv->getNextLine()
  ...datainjection/inc/clientinjection.class.php:226 PluginDatainjectionClientInjection::processInjection()
  ...datainjection/front/clientinjection.form.php:45 PluginDatainjectionClientInjection::showInjectionForm()
  public/index.php:82                                require()

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