Mention data-hotkey in the 15.0 migration guide

In Odoo 14, hotkeys on buttons were assigned automatically. In Odoo 15, hotkeys have to be assigned specifically for any new button added in our modules, using the `data-hotkey` attribute. See https://github.com/odoo/odoo/pull/73275. I would suggest mentioning this in https://github.com/OCA/maintainer-tools/wiki/Migration-to-version-15.0.