refactor: 增加阴影效果

Author: ArgoZhang

src/BootstrapBlazor/Components/AutoComplete/AutoComplete.razor

@@ -23,7 +23,7 @@
 
 @code {
     RenderFragment RenderDropdown =>
-    @<ul class="dropdown-menu">
+    @<ul class="dropdown-menu shadow">
         @foreach (var item in Rows)
         {
             <li @key="item" class="dropdown-item" @onclick="() => OnClickItem(item)">

src/BootstrapBlazor/Components/Select/SelectBase.cs

@@ -171,7 +171,7 @@ public abstract class SelectBase<TValue> : PopoverSelectBase<TValue>
     /// <summary>
     /// Gets the dropdown menu class string.
     /// </summary>
-    protected string? DropdownMenuClassString => CssBuilder.Default("dropdown-menu")
+    protected string? DropdownMenuClassString => CssBuilder.Default("dropdown-menu shadow")
         .AddClass("is-fixed-search", CheckFixedSearch())
         .Build();