[EuiPageTemplate] - Remove shading styles

[JasonStoltz]

The goal is to remove it from KibanaPageTemplate in Kibana as well.

[mgadewoll]

ℹ️ the initial update to update EuiPageTemplate backgrounds styles that @tsullivan did, was updated a bit to consider a more generic, less strict approach by supporting background options via props.
⏳ Waiting to align/confirm the changes match requirements on Kibana side.

The update has been deployed with the underlying changes by @tsullivan on Kibana side on this test instance.