Bump version of shiny to support bslib and align with teal's min shiny version (#367)

Bump shiny version as per the discussion
[here](insightsengineering/teal.widgets#312 (comment))

Author: vedhav

DESCRIPTION

@@ -35,7 +35,7 @@ Imports:
     rmarkdown (>= 2.23),
     rtables (>= 0.6.11),
     rtables.officer (>= 0.0.2),
-    shiny (>= 1.6.0),
+    shiny (>= 1.8.1),
     shinybusy (>= 0.3.2),
     shinyjs,
     shinyWidgets (>= 0.5.1),