don't use backdrop for dialogs (#9649)

war/src/main/scss/abstracts/_theme.scss

@@ -215,7 +215,6 @@ $semantics: (
     --dialog-backdrop-background: hsla(240, 10%, 20%, 0.8);
   }
 
-  --dialog-backdrop-filter: brightness(3) blur(50px);
   --dialog-box-shadow: 0 10px 40px rgba(0, 0, 20, 0.15),
     0 2px 15px rgba(0, 0, 20, 0.05),
     inset 0 0 2px 2px rgba(255, 255, 255, 0.025);

war/src/main/scss/components/_dialogs.scss

@@ -3,9 +3,7 @@ $jenkins-dialog-padding: 1.3rem;
 .jenkins-dialog {
   border-radius: 0.6rem;
   border: none;
-  background: transparent;
   box-shadow: var(--dialog-box-shadow);
-  backdrop-filter: var(--dialog-backdrop-filter);
   animation: jenkins-dialog-animate-in 0.25s cubic-bezier(0, 0.68, 0.5, 1.5);
   overflow: hidden;
   padding: $jenkins-dialog-padding 0 0 0;