Fix KibanaPageTemplate.BottomBar in serverless (elastic#168235)

## Summary

The latest changes applied [here
](elastic#166840) caused the
`KibanaPageTemplate.BottomBar` to not work properly:


![KibanaPageTemplate.BottomBar](https://github.com/elastic/kibana/assets/17747913/5720fb62-1352-4140-b0f6-93c818689660)

The `margin-left` should be applied only to `.euiBottomBar--fixed`
instead of all `.euiBottomBar`, `--sticky` and `--static` are already
positioned correctly to the relative parent, so no margin needs to be
applied.

src/core/public/styles/_base.scss

@@ -26,8 +26,8 @@
   margin-left: 320px; // Hard-coded for now -- @cchaos
 }
 
-// Add support for serverless nabbar
-.euiBody--hasFlyout .euiBottomBar {
+// Add support for serverless navbar
+.euiBody--hasFlyout .euiBottomBar--fixed {
   margin-left: var(--euiCollapsibleNavOffset, 0);
 }