Update template to use bslib

[cjrace]

Is your feature request related to a problem? Please describe.
We've known for a while we want to switch to using bslib, we should do that. shinydashboard is long depreciated and we need to remove the dependency.

Describe the solution you'd like

1. Update the example ui to use bslib, likely through either page_fluid() or page_fillable()
2. Update value boxes to use the shinyGovstyle function and drop the shiny dashboard dependency

Describe alternatives you've considered
None - we need to move to bslib and remove the useShinydashboard reference.

Additional context
There may be some styling issues when the page moves to bslib, we should try to fix those as best we can here too.