[FIX] report: blank page on Background template

The `Background` template report (added in 3d04e4488) has been reported
in some instance to have CSS which could cause an empty page at the end
of the report.

This is caused by the following CSS rule which allow to have the
background at the bottom of the content:

.o_report_layout_background {
	min-height: 1000px;
}

this is probably caused by a difference in header/footer height, font or
version of wkhtmltopdf (this has been reproduced on wkhtml 0.12.1).

A fix is hard to do for this and keeping the background at the bottom of
the report content because of how wkhtml behave. So to mitigate (and
hopefully it doesn't come back) the issue, this commit just change the
CSS min-height.

opw-726546
opw-743315
closes #17072

addons/report/static/src/less/layout_background.less

@@ -34,7 +34,7 @@
     background-size: cover;
     background-position: bottom center;
     background-repeat: no-repeat;
-    min-height: 1000px;
+    min-height: 900px;
     thead tr th, h2 {
         color: @odoo-brand-primary;
     }