Revise AppBar spacer classes and styles

[inikolova]

In the AppBar tests we use k-spacer k-spacer-sized classes for the spacer:
https://github.com/telerik/kendo-themes/blob/develop/packages/html/src/appbar/tests/appbar.tsx#L42

However, the issue for "Unify rendering appbar" suggests to use k-appbar-spacer k-appbar-spacer-sized classes instead:
#4092

The component implementation in jQuery, Angular, React and Vue follows the suggested rendering in the aforementioned issue (i.e. usesk-appbar-spacer k-appbar-spacer-sized classes), however the implementation in Blazor relies on k-spacer k-spacer-sized. We should revise the styles for .k-spacer / .k-spacer-sized / .k-appbar-spacer / .k-appbar-spacer-sized and unify their usage per suites as they serve the same purpose.