[IMP] account: warn msg decimal accuracy on modification

commit 31e1322b is allowing the
customer to increase is decimal accuracy for currency without
consequences but triggering issue upon lowering it if entries
already exist.
+ Remove the redirection to the support.

Let's ensure that the behavior is consistent and the users are
warned on first modification. To do so, adding an alert msg
in addition of the blocking message in case of lowering the decimal
accuracy after already creating accounting entries.

opw-2428857

closes odoo/odoo#64757

X-original-commit: 66533ebd
Signed-off-by: Simon Goffin (sig) <sig@openerp.com>