Replace the gear image attachment in email notifications with an emoji

Context: https://github.com/xwiki/xwiki-platform/pull/3037#issuecomment-2152454838

The email sent with notification uses a picture as an icon. It's not the best because:

• it makes the mail a bit heavier
• we cannot change its color easily to fit the theme

We should find another solution. An icon is probably not possible, we cannot share Font Awesome in mails AFAIK.
This icon is just a gear, so we could use an emoji instead. The emoji is light and would fit the theme of any app the mail is read on.