refactor(ui5-popover): remove hideBackdrop property

[TeodorTaushanov]

Remove the hideBackdrop property

BREAKING CHANGE: Property hideBackdrop is removed.

Previously the application developers could define a modal popover without visible backdrop as follows:

<ui5-popover modal hide-backdrop>

Now the application developers can use the standard ::backdrop CSS selector

<style>
.transparentBackdrop::backdrop {
  background: transparent;
}
</style>

...

<ui5-popover modal class="transparentBackdrop">

Related to #8461