12.0 move `send_mail_with_multiple_attachments` in separate module

[victor-champonnois]

• the method send_mail_with_multiple_attachments from l10n_be_cooperator should be put in a separate modules in addons.
• it could be proposed to the OCA.

We could also use this module, after backporting it to 12 or forward porting it to 14.

@robinkeunen said:
Definitely replace by OCA's module

[github-actions]

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