[FIX] core: use proper break (hr) in PR template footer

A "thematic break"[1] is composed of 3 or more characters (-, _,
or *). The current separator using only 2 characters between the main
body of the PR and the footer / addendum makes it more difficult to
interpret the message.

Update to 3 so e.g. the mergebot can strip the footer.

[1]: https://spec.commonmark.org/0.30/#thematic-breaks



closes odoo/odoo#101072

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>