build(gitlab): add mr template

Template

Doc: https://docs.gitlab.com/ee/user/project/description_templates.html#create-a-merge-request-template

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CHANGELOG:

### XXX
- [#nn](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/nn) Example text

Related card: https://trello.com/c/XXXXXXXX/xjksdjflsdfksldfsdfs

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